I recently sold my 2007 XC90 and had done the LED tail light swap upgrade. I ordered the full LED replacement kit that included the resistors which I attempted to install but still had issues with the Position Bulb warning light. I got used to clearing the warning light every time I started up the car for a few years because I still preferred the look of the LEDs
Anyway, I put the old original tail lights back on before I traded the car in to get rid of the code, so now I'm looking to sell the LED units if anyone is interested in the upgrade (or if they need to replace theirs on later model years). Maybe you'll have more luck getting the wiring right.
PM me if interested. I'd prefer to sell locally in the Seattle area to avoid any damage in shipment. Asking $200 for full set including resistors.