I added a keyless entry system to my car, and it was easy, that is once I figured out where everything goes:
I bought a $30 kit from ebay:
the front door was super easy, only had to drill one hole, its highlighted in orange.
the screws straddle the oval hole perfectly
back doors werent bad, either. I had to drill a hole in the bottom or the door to run the wires in and heres the solenoid mounting, drilled holes in orange:
units
anyway its a cheap and easy option for an upgrade or replacement of faulty original lock solenoids.