So, yesterday I'm on here whining because my 2004 Passat doesn't come with red lighting in the dash vents. Seems as though this was a nice option that was discontinued in the 2003 model year. Well my friends, not to be outdone by an electrical mod... I went to work on it this afternoon.
STEP 1: Pop the vents out of the dash.
The best way to get in there is with butter knives. You need to pry up on the actual dash (circled above), and pop the tabs out (circled below) and pull the vent out. There are tabs on the top and bottom of the vents.
STEP 2: Remove the cover from the vent.
STEP 3: Solder a 680ohm resistor to each LED (there are 8 LED's in total).
I used 2mm LED's, but in hindsight, I would recommend using the larger 5mm ones. Here's what to buy:
STEP 4: I used a hot glue gun to hold the LED's in place.
STEP 5: Pop the cover back on.
STEP 6: Test the lights (I used a 12volt battery charger to test).
STEP 7: Pop the vents back in, and wire them into your headlight switch. The negatives of the LED's can all be wired to the chassis ground of the car. The positives all get wired to pin #17 of your headlight switch.
STEP 8: Viola! Enjoy the coolness of your new mod.
Total cost of parts: $7.25 (Radio Shack) Total labor: about 2.5 hours. Total coolness: priceless...
UPDATE: I've had a number of people PM me asking how to get the vents out of the dash... They're a bitch, but once you get one vent out, you figure out the technique and the others will be easy. On the small vents there are 4 tabs (2 on top - 2 on bottom). On the center vent there's 6 tabs.
Basically, you use butter knives to pry the actual dash upwards, and then pull the top of the vent out toward you. Then do the same for the bottom.
The dash is made of a real 'spongy' foam material. You can bend the shit out of it with the butter knives, and it'll return to its original shape. Don't be afraid to really pry on the actual dash.