The key fob receiver is built into the passenger side multiplexer behind the fuse panel on that side. It has no connection to the radio antenna. You seem to have covered the usual suspects, new fob battery, cleaning connections, etc. Since you had recent work on the motor mounts it would be logical to look for wiring problems in the engine compartment. Otherwise you may want to pull and inspect all the panel fuses for corrosion. Last resort might be to replace the passenger side multiplexer.