Im looking for mods to do to a P-Bass, that have to do with a switch to turn them on and off. (Or plans on how to build an easy sustainer.)
Well I don't play bass but I know stuff so...

I wouldn't bother with a sustainer, on a bass it seems pointless and they have a long sustain anyway. Remember you would need something that goes first in the effects chain so maybe compression or wah? More buttons and you could go for overdrive or something with internal settings so your bass isn't a bid dial. Go Crazy!
maybe an onboard preamp? wouldn't be too hard, there is a really good book for this sort of stuff called 'guitar electronics for musicians' its a bit old and some of the stuff is dated, but its got really really good info, buy it yesterday

unless you want to go active, which is probably the best idea, there isn't alot you can do with one pickup

if you go active, the possibilities are endless, rip out some pedal circuits, put onboard preamps and EQ's..... do what he ^^ said, go crazy!
How would one go about building an amp into his guitar (You know a small one) Would you need a preamp, or jsut a switch to change your power over to the speaker or what?
would you want a speaker in the guitar or just an amp in the guitar?

if you go to maplin (UK) they sell kits for 5/7/10 (up to 200)w audio amps that are just straight up with no controls, you can probably get them elsewhere

just have it between the pups and the jack and run it through a cab with the right impedance, and it'll drive it, i'd put a switch that will bypass it incase you wanna drive another amp