I started writing a post on this earlier and got sidetracked, so here it is below (a little late as you may have already gotten the info you wanted). Apologies for the hijack, I suppose somebody can move these if that's better...
The camera is a cheapo off ebay for like $15, but I guess you could spend more and probably get a little more. Mine has worked since October, and it's been through plenty of weather so far. I have an identical one sitting around for when this one dies (for $15 why not?). Actually I think this is it: http://r.ebay.com/JsgKPY, maybe the price went up a few bucks.
To make the camera work with the rearview mirror, you have to splice into the Tacoma's system somehow -- unless you run wires up to the cab. I wasn't comfortable messing with my brand new truck (I'd never done anything like this before) so I got an extra wiring harness. An old email says that it's part 82182-04010 from ToyotaPartsZone.com, price $30. When you take off the tailgate, you easily unplug this wiring harness from another set of wires in the frame of the truck (instructions are in the owner's manual for removing the tailgate -- it's so easy that even I got it on the 1st or 2nd try...). I believe this part is the same for all 2nd Gen Tacomas, but I'm not sure.
The camera is a 12v camera, so I powered it from the reverse light. Then, you've got to connect the camera's video to the wiring harness. There are 4 wires in the harness, 2 for power (which I didn't use -- the Tacoma camera is 5v or 6v as opposed to most backup cameras which are 12v) and 2 for video. My camera's output is Yellow Video or RCA, so I snipped off the yellow plug and connected the two wires inside to the appropriate wires in the wiring harness. Plug in the harness, put it in reverse, and ideally you've got video. I got the colors of the wires off TacomaWorld.com:
http://www.tacomaworld.com/forum/technical-chat/226770-backup-camera-help.html
You can also get an OEM Tacoma camera and skip the splicing, but I didn't know how to mount that as easily as the eBay camera, and I wasn't sure if it'd stay dry if it wasn't in a well-built housing. And those cameras cost $60 or so, so I didn't want to learn with more expensive parts.
Note that the gridlines are in the Tacoma's mirror (in my case at least) so if you buy a camera with gridlines, you'll have 2 sets. They also won't be perfectly straight unless you are perfect at installation.
I don't know which RCA wire goes to which wiring harness wire at the moment. I can try to dig that up if you have a hard time finding the answer. Basically one is positive video and one is negative video, and I had no clue what that meant at the time, but I probably made a note of it somewhere and could figure it out.
