For an in depth discussion on the lighting characteristics of the caprice taillights and the instalation of Sequential taillights see Marv Davis's site where he sells the Sequential taillight kit I have installed on my caprice and could not be more happy with. He goes into much more depth than I did on this page.
http://www.sequentialtaillights.com/
This is one of the first modifications I did to my 91 Caprice.
The taillights on these vehicles are so huge that not having every part of them
lit up is a waste. I feel after the modification the taillights look better
and they are safer due do having a larger area lit up. Below
is the rear view of one of my taillights. On it you will notice two things compared
to a stock taillight: 1. The large hole in the top center
pod. 2. The extra smaller holes along the bottom on each
side of the back up light pod.
Removal of taillights: Inside of the trunk, to the outside of the vehicle, there are two plastic nuts that hold the taillight on. These nuts also double as the cargo net holders (if equipped). Unscrewing these loosens the outside of the taillight. Inboard on the taillights there is a 10 mm bolt that goes into a tab coming off of the taillights. Once this is removed the taillights are ready to come off, except for the wiring. Taking out the bulbs is easy enough all unscrew counter clockwise in some fashion.
Third Brake Light In Taillight: On stock Caprices and Impalas when applying the brake or turn signal only two of the three reflector pods get lit up on each taillight. I thought it would be better if they all did so I looked into adding a dual element bulb to the center pod. After taking off the taillight it looked to me that GM had originally planned on doing this but cheaped out and just put a single element bulb in there. I thought I would run into a problem with the pod not being big enough for a 1157 or similar bulb but that was not the case. The center top pod is huge (as you can see in the photo, the one above the backup light pod) and can easily take an 1157 or similar. I decided to use a bulb socket that is commonly referred to as a universal socket. It is a dual element socket that takes 1157 or similar bulbs. It's mounting mechanism consists of a series of metal teeth that grip onto the edges of a hole that you make. I enlarged the single element hole in the top center pod of my taillights to a 7/8" hole using a conical file bit in my drill. Get your bulb sockets before you make your hole as the size may vary with different manufactures. When installed the rim of the bulb socket touches the slope of the taillight plastic coming down to the hole. As I live in California and do not get much rain I thought that there was no need to seal up the small gap that water may get into where the bulb socket meets the taillight. Others in more harsh climates have gone to home centers and gotten rubber o rings to insert around the bulb socket and that has done the job.
Lower Reflector Bulbs: Once I had the dual element bulb in the top center pod I had an extra single element bulb that was just hanging there. I decided to see if I could put it along the bottom row of the taillight that was basically just a reflector. It looked pretty thin, like the bulb might touch the back of the red reflector but i decided to give it a try and there was enough clearance. On the lower row of reflectors there are no holes pre drilled so you have to make them yourself. I drilled three 5/8" holes along the bottom row being careful that when the bit went through the back of the taillight it did not scratch the inside of the red lens (do this step on a drill press if you have access to one and set the depth adjustment accordingly). I then used a 3/16" square file to make the square protrusions to allow the socket to slide in (put a paper towel in the tail light when doing this so the end of the file does not mar up the back of the red lens). After about two holes of making the four protrusions I figured the bulb would stay in there fine with only two protrusions so I started cutting off two of the tabs on the sockets (opposite each other). It's easier to cut the soft plastic of the bulb sockets than to file the hard plastic of the tail light. The bulb sockets I used are almost exactly the same as the top center single element bulb that was replaced with the two element bulb. Bought them at my local parts store. A basic description of them would be they go into a cross shaped hole and turn clockwise and lock into place. Two of the tabs on the sockets lock and two are just there to hold it in place, make sure and cut off the non locking tabs if you are going to cut them. They come as just a socket with 2 wires coming off of them about 6" long. I did not check if GM offers them individually. Wiring Up The Taillights Basically you just tap into the wires that are already on the taillight. Just make sure to get the bright element of the new brake/turn signal socket into the right wire off of the harness. Some have reported that when adding the new brake/turn signal bulb the flasher needs to be upgraded to a heavy duty one to compensate for the extra load. If your turn signal does not work properly after the mod that is most likely the problem. In my 91 Caprice I did not have to change the flasher but others with 94+ Impalas did.