I had these sort of installed for a while, but I thought I should go ahead and make it a bit more permanent. Still need to replace some of the chrome and brass hardware with black pieces.
Tekonsha P3 brake controller and ScanGauge
iPod holder and Garmin Nuvi (don't laugh at the over-size iPod holder -- I crushed my old 30GB and just recently replaced it!)
The ScanGauge does what it does very well. I bought it to keep track of some more information while towing, but it turns out that getting data like transmission fluid temp is still not possible. It also reads and clears trouble codes, so it is useful. I do like the instantaneous MPG and horsepower display (mostly just curiousity at this point). Seems to be a bit more accurate than the multi-information display in the 4Runner. Some folks on the 'net are working on the custom commands to get more Toyota-specific data out of the ScanGauge. Should be more useful as times goes on...
Install is easy. Plug it into the OBDII port. Configure your engine displacement and it starts working pretty well. Fine-tune the fuel consumption by clearing the tripmeter on a fill-up and entering the number of gallons added on the next fill-up. Pretty straightforward. It comes with good documentation. The X-Gauge part of the documentation will tell you what data is available for display. Folks in the Tundra forums are more up to date on custom programming.
I'm assuming your username is an indication you are a Cub pilot, right? Just asking because I am taking the first step getting back into flying this weekend. I got all the way to the checkride, then moved across the country and never finished! Found a co-worker who is a CFI-I with a nice PA28. Going flying this Saturday.
Should have mounted it right on the fuse box cover (Just below) so it could be removed easily when not using. You then get another cover from Toyota and back to the OEM look.