99 VW Jetta Brake Lights & Temp Gauge

ok, i need some help on this one guys. a couple of days ago another installer put an amp in a 99 Jetta using the factory radio. last night (wed) the customer brought the vehicle back to the shop saying that the woofer, temp gauge and the brake lights were not working. i called the installer that did the vehicle at home to see if he might have an idea what the problem could be but he said that he didn't really remember anything out of the ordinary except that the radio had coded when taking the battery terminal off. the funny thing about the coded radio was that when the customer got into the car and cranked it the radio wasn't coded any more and everything worked fine.

when i went to check the vehicle in last night the customer's dad told me that he saw the brake lights started working when his son turned the corner into our store. when we went out to check the temp gauge it was working as well. as i began working on the vehicle i noticed that there used to be a remote start in this vehicle but it had been removed (this customer has only owned this car for 2 days prior to us working on it). as i tested the brake wire i noticed that the brake wire at the brake did not change from ground at any time while testing it and then all of a sudden i lost the ground completely. then i diode tested the wire with a DMM and noticed that the only time i could get the diode test to break continiuty was when i reved the engine while pressing the brake. it seems like the ground in the vehicle is being lost somewhere or there is some type of voltage loss. also, when checking the battery it shows 12.15 to 12.18 when cranked or without the vehicle being cranked. any help on this would sure help and maybe save a investigative damage claim. thanks in advance for all the help.