*lifted from the vortex thread I posted earlier*
(Ignore first bit, we already have the 4 wires. US cars have 3 and have to add a wire to the unit.)
VAG-Com Instructions
Once you have connected wires to all four pins and reinstalled the connectors into the taillight housing you are ready to recode your central electronics module. This coding appears all over the Jetta LED threads, so I don't know who to credit for this information. Using a VAG-Com, you need to make the following changes to the Central Electronics Module (09).
Change Bytes 9 through 13 from 14 00 00 00 14 to 0A 00 00 0F 00
Change Byte 18 by unchecking the cold diagnostic bits 1, 3 and 5
Change Byte 19 by unchecking the cold diagnostics bits 2 and 3