Your best bet is to get a qualified fitter to do the job, unless your an auto electrician! you have to be especially careful on much newer cars, because of the nature of the engine management, and fuel management system wiring, you could do a lot of damage to the ECU.. all for the sakes of a round of drinks! Older cars are not too bad, though you might run into problems with existing alarm/immobilizer wiring, again, unless you know exactly what your doing, I wouldn't bother, shop around for a good price though
Tom