My advice? Buy something close and modify it to work - I had to do that with a Bally Playboy that had a rectifier fire in the head. Went through the harness, had to switch a few wire positions on the donor to match the PB manual but in the end it worked great.

In the case of FG you'll need to run a few extra for the strobe, but beyond that most any 7-digit game harness should get you going. The arcadeboneyard has one from a Black Pyramid that should be enough to get you moving ahead on this.

You may end up waiting a long time for a game-specific harness to show up. If you're set on 'originality' you could always keep your eyes out for one while putting in a donor temporarily.

You would need to double-check the harness wiring from whatever donor you use against the manual to make sure the wiring is correct. I my case, there were a few I had to move.

It took me a few hours to go over everything, but I was rebuilding all the connectors as I went so obviously it took longer than it would have if I was just checking...

Not sure - if 6 digit I believe you may have more work to do. Again, you need to check the donor harness wiring against the manual to make sure every wire is correct and move any necessary.

Nobody said it was going to be a 'plug-n-play' situation using a harness from a different game. It can be modified to work, if you are willing to put some time and effort into actually doing it.

The BP and FG have way more things in common that differences, so it can be done. Download the manuals for both games, go over the schematics (which tell you what what pins\wires go where) and make your adjustments. I recommend rebuilding the connectors as you go, not only for ease but bally connectors are notorious for being flaky.

Only real differences I noticed are:
strobe (already mentioned before)
location and model of aux lamp driver (giving you more wire than you need - since on FG it's mounted on the same board as the displays where BP it's located way over by the sound board) You are going to need to change connectors here anyway so rebuilding the rest as you go is again recommended.

It can be done, you just have to be willing to put the work into it - or you can just go back to waiting for an actual FG harness to eventually come up for sale. Up to you...

Glad to see you are willing to try!

I'm sure others may have some suggestions, but here is a simple rundown of what I did and would recommend to you.

Start by determining what connectors on your donor harness go where (like to displays, lamp, mpu, etc) and noting where the connector fits (A4J4, A1J1, etc.) On each I used a piece of tape, wrote what it was called and wrapped loose around wire close to connector.

Then I printed the Plug Connectors Diagrams from both the manuals, compared them and highlighted any and all changes, noting what needed to be changed and where wire needed to be moved to (or even added). Your biggest changes will be shortening\moving the aux lamp run and adding the strobe wires, which you can possible re-use some of the cut-off wire. It also appears that FG uses more lamps than BP, so some additional wires will more than likely need to be added.

Using that, I went to work rebuilding the connectors one at a time and made the changes as I went.

Tools and parts needed:
All the proper connectors\pins\crimper (greatplainselectronics.com has everything you need)
You're most likely going to need .156 pins\connectors for displays and aux lamp but you may need some .10 for connectors to lamp\mpu. youtube has some nice videos demonstrating crimping if you've never done it before. Believe me, it's a skill that you will really use to maintain any pinball machine.

I also bought a cheap 'continuity tester' (which is just a light bulb with battery and leads) to make sure that the wire I had at one end was the same one at the other (sometimes just going by color alone will get you into trouble)

The obvious wire cutter, wire stripper, zip-ties, etc.

It's all a bit of a learning process, but when you're done you will really feel great about the whole thing. Any questions, ask away, and I'll try to help.

I would say go ahead and remove the old one - but take some detailed pics for reference in case you have questions later (never hurts) If you haven't already done so, now would be a good time to clean up that insert board.

I would then size up the donor by connecting displays, noting route of where donor harness wants to 'lay' and marking your connector\board locations - without powering anything on. Then remove donor harness and start with printing out the plug connector diagrams (11x17 paper works great for this) and making notes on what needs to be moved\changed\added.

For me, I laid the donor harness out an a large chunk of cardboard on a large flat surface that I could use for several days. Cardboard was nice as I could make notes on it as I went along (like how many changes were to x connector per diagram, etc.) I then placed some new zip-ties close to the old ones - not making them tight so I could then cut all the original ties off and leave room to adjust and pull wires through.

At this point, if needed you could lay your old harness across your donor to get an idea of where the aux lamp connectors and strobe need to be and how long they are, etc. and mark notes on the cardboard.

Then it's just a matter of choosing a connector and starting.

Sounds like a lot of work but it's not. Just work at a comfortable pace and you'll be done in no time.