It doesn't really sound as though you have the premium sound in the truck at present. You probably do not need a custom install. BUT, there is a way that you can find out. You DO need the harness, unless of course you want to cut out the existing plug. Once you have your harness plug it in without it being hooked to the stereo. Then you need a 9vt battery, take your speaker leads off the harness + and - to each speaker, touch these leads to the 9vt, you should hear a pop and static at each speaker location. As long as you hear this sound you have no amp. If you do not hear this sound then you would need to run speaker wires to your present speaker locations, this translates into alot of work. You have to remove mouldings ETC. It's really not that hard just takes awhile. I had to do it with my 4 Runner. I assume that you have speakers in the doors and the rear somewhere. The doors will be the hardest to run as you have to remove the door panels and run the wires thru the door jam and into the door. Putting in a stereo is not hard even if you have to cut the factory harness, all there is is a hot (b+) (RED) a common (-)(ground, BLACK) and an ignition hot(+)(YELLOW). Then you have the speaker wires. Unless you are installing an aftermarket amp, then you will have to hook up the blue wire off the stereo (REMOTE).THe orange is for the power ant if you have one. These colors are usually standard, but you will have a wiring diagram with the stereo. If you have any questions feel free to email me. I can help you out.
Searcher (1986 sound challenge 1100 watt class winner) IASCA Yeah its been awhile since I was into the boom cars but I still keep my paws in the loop, I am no longer into the BOOM but I do like HIGH quality sound. and by the way a Pioneer head unit has been my top pick since 86, I have never had anything else win in my cars. I run a Pioneer 6700 with Infinity reference seps. Getting ready to hook it all into a Rockford Fosgate Punch amp.
goodluck