Radio Relay Trick
Thanks to Steve #1 for contributing this handy article. This how-to explains how you can wire in your radio so that it stays on after you shut your truck off and open the door.
This article is reposted from FordF150.net, we are unsure of exactly which vehicles this will work on but at the time it was written it worked on certain F-150s (1998-1999).
Use these relay systems to keep your head unit on until you open your door.
Using one of these two relay pairs will turn your head unit on with the key as normal, but the head unit will stay on until you turn off the key and open the door. Be sure to use the top diagram for negative door triggers and the bottom one for positive door triggers.
DISCLAIMER: Check all wires before making connections, and do not hold me responsible for anything negative that may result from you following these instructions.
This is a fairly simple procedure, but it is always wise to check your wires using a multimeter, before you make any connections. You will need two 30 amp relays, I recommend Bosh, or Potter and Brumfield, but any will work. You will also need a 10 amp diode. I once used a 6 Amp diode and did not have any problems if you are unable to find a 10 amp. You will also need to solder and heat shrink connections, or use crimp connectors and about 10 feet of 16-18 gauge wire. All the Fords I have done have had positive trigger door switches. In the 98 and 99 F150 it is a Black/Blue wire in the drivers kick panel. I have a wire code sheet here for the 98 and it also says that on the base model the door trigger is negative with a Orange/Green wire.
Article contributed by: Steve