That all depends on your patience. I have added a $2.50 fuse block I found at Radio Shack and I now have electric wipers and power seats in addition to the heater, lights, signals, etc.
The basic harness accomodates turn signals and essential funtions, but that's about it. All accessory items have to be wired individually. I have been doing this for years, so it's no big deal to me; if this seems like a daunting task look for a more sophisticated harness. Remember that every electrical device needs to be fused on one end and grounded at the other. The rest is just "going with the flow."
