First time post here so I hope this is in the right spot.

So basically I'm stuck in fourth or fifth gear.
This happened about a week ago and I am trying to figure out how to get it fixed.
I am not sure if its related, but my friend put in a new subwoofer and 2 days later my transmission isnt working properly. ( ran a 6 gauge wire from battery, through firewall under passenger seat to the trunk, put a ground on my back seat bolt )

I found out my car was messing up when I went for a drive, came to a stop and then put my foot on the gas and had a very poor response due to it being stuck in a high gear. Also noticed that the dash light symbol for D, R, N was out. Anyway, I drive it home, and i tired to shift it manually using the manual shifter option but no luck.

The next day I drove it to autozone for a code check, these appeared:
p1730 A/T Interlock
p1762 A/T D/C Solenoid Valve Circuit Malfunction
p0720 TCM Revolution Sensor Circuit Malfunction

Took it to an auto shop. Paid 150 for him to tell me something is wrong with the wiring harness that goes into the internal components of the transmission. Thinks* he can fix it by replacing the wiring harness and flushing the trans fluid. Wants 950 for this.

This kinda sucks, the car has 105k miles on it and i feel like i could almost find a used transmission/ rebuilt one for cheaper. If anyone has some pointers i would appreciate it.
Also where is the best place to purchase a reasonably priced rebuilt transmission for g35.
Thanks guys,
Alex