2. take the phone off via the two lock nuts, then remove the mounting plate, then get your fingers in there

Yeah it's funny this question came up since I just replaced the flasher in my phone yesterday. The lock nut at the base of the phone in front needs to be removed, and the one behind the phone only needs to be loosened because then the plate will slide out because it is slotted. That is good because it would be impossible to get that nut started again with the phone in place with no room to fit your fingers behind it.