Well... The last vehicle I wired speakers up too was my old 1992 gmc dually...
Now.. out on the pipeline (which is where I work) I and all the other welders would have speakers running out of there cabs. Basicly what you do is wire home entertainment speakers and drap the wires out the cab and onto the deck..
-Could this potentially start a fire or burn up my head unit on my new work truck? Theres some freaky myths and legands out there... And I know the pipline welders arnt the first people to do this..

generally as long as the speakers resitance pairs up with what the deck can run, than you should be ok......

Well.. these arnt ment to have good quilty.. just to make work feel less like work... and a 10'' with a 3.5'' home entertainment box is just right...(also the price... zero dollars) but It could be expensive if i burn up the stock cd player or wiring in my 03 chevrolet dually work truck...

how would I go about finding the reistants, and how much of a variation do I have?

you shouldn't really have a problem. Most home audio components are 8 ohm, most car audio is 4 ohm. It should work fine as long as you're not cranking the speakers up too much and over working the head unit.

sounds good, always good to have music while you work...
I'll try not to crank them up to bad, generally when the start to loose bass is where the dial stops..

Just watch out how much power you dump into the speakers. After all, speakers in an open environment sound nowhere as loud as in your living room, and especially with machinery and everything creating noise elsewhere, it can be all too easy to inadvertantly crank it up a bit too far.