After having a new stereo put in by a Ford mechanic ive lost rear tail lights and signals. new flasher and all bulbs are good. replaced the turn signal switch and the headlight switch.No help
bring it back for him to fix. you paid for it, he should make it right.
This may be a little simplistic, but did you check the fuses?
Shop may have inadvertently disconnected the tail light circuity while tapping into the wiring harness for the stereo install. I expect this will turn out to be a minor problem. Good advice above to ask the shop who did the stereo install to assist.