Does anyone know if the security system wiring carries enough amperage/voltage to piggyback a keyless entry system off the connector while the security system is operational?

 

Yes i've installed an alarm on top of the factory system before and it does seem to work

However I would suggest you look into unhooking the factory system and just use an aftermarket one.
Since the sensors are already in place (you can tap the wiring from the factory Alarm Module) it isn't too hard to install aftermarket alarms (or maybe you want to tell your alarm/car audio shop to do it)

Btw, if you don't have keyless entry already, you need to buy at least 1 door actuator to control the driver side (the passenger side is a pick-me up type actuator, meaning that when the driver side lock goes down, the pass will go with it)

 

I went ahead and made a new bracket for the factory pick up actuator and just ran it on the other side. Ran the wiring in the loom. All the holes are already on the door and the latch to use it, plus I've never really been happy with the awful screw down hookups you have to use on the aftermarket ones.

It's just and Ungo standalone keyless system, I'm really pretty happy with the factory alarm. There was an aftermarket in there when I got the car, but that lasted until it went off while I was driving.

Thanks for your help, now I just need to make the piggyback loom.

 

Are there actuator brackets in the passenger door of S4s or only S5s?

 

I got mine out of an S4 TII, I think it is only in the late 87, 88 S4's because mine is a February 87 and it doesn't have the system. Without the wiring your kinda stuck if you wanted to add it. You would need the rear car loom as well as the dash loom, the interior and exterior door lock switches, and the actuator relay to make it work.

I picked up the relay in the hopes of just keylessing the drivers door, but I'm not going to replace all that wiring just to do something I could still do with the aftermarket keyless system.

It's pretty easy to make the bracket, I made a pattern out of the passenger side. I got a sheet of stainless, cut it with a jig saw and bent it with a vise break press. Welded some nuts on and screwed everything together. You have to bend the rod a little bit, but other than that it's a perfect fit.