Not co-incidence i think.

I wonder if the audio out from the ULF is interfering with the cars audio system, it might be an idea to see if the audio wires are not pinched or crushed (from ULF to the DB25 plug, and that they have a good connection).

You may need some sort of choke to suppress any stray interference
being carried down the lead from the ulf.

__________________
Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.