Hard to tell from the picture, but are the GI lights behind the backglass also out?

If your playfield GI lights AND your GI lights behind the backglass are out, it is likely that Fuse 5 on the rectifier board is out. If this is the case, you likely have a short in a bulb or socket. See pics below if you are unfamiliar with the rectifier board (also referred to as the power supply).

Fuse F5 - General Illumination (G.I.) Fuse Woes.
There isn't much to this circuit, so if fuse F5 blows, this usually means there is a shorted general illumination bulb or socket. This is never a quick or easy fix - you'll have to do quite a bit of looking and eliminating to find the problem.

First, a good idea is to purchase a clip-on circuit breaker. Instead of replacing the F5 fuse for each test "power on", the circuit breaker can be reset and reused. This is great for G.I. problems and saves lots of money on fuses. Just clip the breaker onto the rectifier board's fuse clips with alligator test leads. A mini circuit breaker can be purchased from any lighting store.

To issolate the G.I. problems:

Remove connector J1 (playfield) and J3 (backbox) from the rectifier board, leaving J2 (cabinet wiring) connected. Power up. If fuse blows, there is a short in the main cabinet G.I. wiring (probably the coin door lamps).
If fuse doesn't blow, remove connector J1 (playfield) from rectifier board, leaving J2 (cabinet) and J3 (backbox) connected. Power up. If fuse blows, there is a short in the backbox GI wiring.
If fuse doesn't blow, remove connector J3 (backbox) from rectifier board, leaving J2 (cabinet) and J1 (playfield) connected. Power up. If fuse blows, there is a short in the playfield GI wiring.
Each time plug J1/J2/J3 is removed, that part of the G.I. circuit is removed. What ever plugs are left connected are the wiring sections being tested. If the short is in the cabinet wiring, this is easy to fix. Just examine the coin door lamps. If the backbox wiring is the problem, this too is fairly easy to examine. A very common problem here is the ground braid that connects the head to the backbox. This can bunch up and touch one of the lamp sockets on the back side of the insert (display) panel (when the insert panel is closed). Unfortunately the playfield G.I. is the most troublesome section.

Now that the offending section (playfield!) has been isolated, it is time to further isolate which strand of lamps has the problem. There are two G.I. lines in the game- red/white wires, and orange/green wires. Now find a strand (either one), and de-solder one of the lead wires to the strand (thus taking the strand out of circuit). If there is a double wire (double green, orange, red, white) on the strand, be sure to keep the double wire connected together once it's removed from the strand. This lets other strands "downstream" continue to have power. The basic idea is to disconnect a strand, power up, watch the fuse (or breaker), and repeat until you find the offending strand.

It's never easy or quick to find a problem like this, but this is about the only way to systematically find the short without pulling out every bulb or looking at every socket/wire.

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What color is the loose wire? Can you post a picture of the area where you found it?

The rectifier board is the small square printed circuit board with all the fuses on it (see picture above). There are 3 connectors that attach to this board. J1 is the top left. J2 is the top right. J3 is the large one on the bottom (underneath J1 and J2). The J1 connector attaches to the playfield and one of its main functions is to light up the general illumination (GI) lights on the playfield. Make sure the following wires, connected to the J1 connector, are properly attached and making good connection to the male pin that is attached to the board: pin 1 - solid white wire; pin 2 - solid green wire; pin 5 - solid orange wire, and pin 8 - solid red wire.

Not only should you replace the crimp contacts and female connectors, but you should also replace the 35+ year-old .156" male header pins that are soldered into the rectifier board or you will have more problems sooner than later.

Crimp contacts:
https://greatplainselectronics.com/products.asp?cat=80

.156" female connectors:
https://greatplainselectronics.com/products.asp?cat=86

.156" male header pins:
https://greatplainselectronics.com/products.asp?cat=79