I installed the Volvo alarm for my 1995 855. My car wasn't come with the alarm nor the keyless entry. Dealer installed an aftermarket alarm/keyless entry system for me. I took out the aftermarket alarm/keyless entry system and put back the Volvo alarm/keyless entry system.

As Gerald stated, you can just buy a key fobs to have the keyless entry. However, in my case, I also need to buy the receiver for the keyless entry. You can check whether you have the receiver. It is located next to the glove box next to the passenger side door. You need to take out the glove box compartment to find out whether there is a black square box in that area. There is a green connector plug into this black box. That box is the receiver.

For Volvo alarm, first check whether you have the "big" alarm relay in the relay cluster under the dash (driver side). It is a 2 relay set. I forget the relay number. The relay set is the heart of the alarm. If you don't have the relay set, you will find a single relay plugged into one of the alarm relay location. The single relay is to connect +12V to starter all the time. Volvo calls this relay a starter bridge. Alarm relay will control the +12V to the starter. You also need to buy the status LED right under the windshield, the siren located in the middle of the firewall, and the driver side hood latch (with wire/connector coming out). All the doors and trunk latch are already prewired to the alarm relay set. No tap-in or anything is need for the Volvo alarm. Volvo alarm is a basic alarm system and can't be compared with the fancy aftermarket alarm systems out there.

Ching-Ho Cheng
--
1995 855 NA, Bilstein Strut & Shocks, Volvo Protective Grill, K&N Filter, E-Code Lens, Alpine CD-Changer, AutoDim RearView Mirror

If you only want the keyless entry, sans alarm, there is good news. According the Rich K (Mr. Bay 13):

"If we are talking a USA car, the car is set up to have remote keys, all you have to do is get the key fobs, and then program them to the car. Volvo also makes the alarm kit to be added if you want."

Gerald C

The Haynes manual for the 850 lists all the wire color codes for the harness. There is a page in the book that has the power door lock relay schematic that has all the wires you will need to connect to (only a few).
I put in a Code Alarm last year in my '96 and was able to get to all the wires I needed (including lights and door switches) in the driver side footwell on the lower left side of the dash. You will need to remove the plastic cover under the dash. The harness is right there.
A helpful hint is to remove the power lock switch on the dash (pop it out) and look at the wire colors coming from the switch. Match these to the ones in the book and away you go. Most of the wires you need go to that switch.
Good Luck

I dont know if this helps or not. Way way back I used to put the alarms with keyless entry systems into cars and particular vehicle schematics for wire were not available since there are so many different types of vehicles. The manual will say what to look out for with voltages. So get a voltage tester with a light and test the wires that go through the door jam. So when you engage disengage the door locks it will trigger the light. It'll probably take some time because of the tightness of the wiring harness. Get a tester than pricks the insulation or better yet find a wiring connector box.