Sorry nobody has chimed in as of yet. Have you checked for power at the input of the TT park lamp relay? I know you said you checked the fuses, but, was that with eyeball or with a meter? Did you check them in place, looking for 12VDC on both sides? If that all works out, then start chasing the wire, physically, down the frame rail, looking for damage or a break. If nothing, then a sharp probe with your meter, looking for where the voltage drops at. conversely, you could pony up and buy a signal tracer.
https://smile.amazon.com/dp/B00ADHQC...v_ov_lig_dp_it