Has anyone installed a DIY Rattler Security System? If so, at what location did you connect (solder) the door lock / unlock connects? The doc says passenger door harness. What type door lock system does the 89 Max have?

 

all 3rd gens have a negative door lock system (meaning you just give the correct wire a ground pulse to activate it)

Since you have an 89, you dont have a door lock timer so it's a pain to get the lock/unlock wires.
You'll need to remove the fuse box (one screw and one clip) and reach your hand into the hole going to the door. You'll find a harness that is 2 rows of 7-10 wires. You'll need to pull this a little to get some slack to make connections, but dont pull too hard so that you pull any wires out of the connectors.
Lock is grey/red
unlock is sky blue

 

What about the parking lights? The Red/Yellow wire on the steering column is 0.0 at OFF position and 11.6 at ON position, however, adjusting the dimmer switch causes the voltage to fluctuate +/- 5volts. According to DIY this is the incorrect wire.

 

Which switch are you testing the wire at? It really shouldn't be a problem anyway. Your security system is using this line to GIVE power to the parking lights, not using it as a power source. So it will send 12V to this line and will power the parking lights to flash them. Just try it, it'll work.
Enjoy

 

Thanks for the help Craig B. Have it installed and all is working well. Except, with the OEM keyless entry, after unlocking the doors by the combination, it doesn't disarm the Rattler Alarm. What's really strange is when I unlock the door by key, the Rattler disarms. Any ideas?

 

that is really weird, does the rattler have disarm/arm inputs? if not, then it may be a pain to wire it up to disarm unless you use the remote