Reddit reviews Docooler Car Remote Central Lock Locking Keyless Entry System with Remote Controllers

For what it's worth, I've had good luck with this unit: https://www.amazon.com/gp/product/B006QH9C5A/ (Installed in a 2001 XJ)

4 years with it and have had no issues, decent range (30ft at least); just be sure you can route the antenna wire as high as possible within the dash or range will suffer. If you take the unit apart it might be possible to fit it where the old module was, and you already have all the connections needed there.

The cheapest and easiest solution is to install an aftermarket keyless entry. or a car alarm, being your car a '97 it should be relatively easy to do it yourself.

Get a cheap universal one off ebay for $15. Tap into your trunk wiring to the solenoid. It will piggyback to your factory central locking (assuming it works).

Minimal wiring and can be removed in minutes

Edit:

Here's one for $16 shipped from Amazon (prime).

https://www.amazon.com/gp/product/B006QH9C5A/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

You can find writeups all over the interwebz

http://zoso.no-ip.org/318i-ke.html

Yeah. Here's what you want

• Door lock actuators
  
  • When you pull the door panels, you'll see a rod that moves back and forth to push the lock on and off. You hook these up to that rod inside the door and run the wires from this actuator to a spot under your dash where you'll install the brain.
    
• Power door unlock 'brain' receiver/transmitters
  OR
  You could put a full alarm system in - though this is obviously more complicated. I'd recommend any 'directed' system, only because that's what I'm familiar with.
  
  Obviously the full alarm would be more complicated to install, but only marginally. There's also the possibility you clip the wrong wire and mess something critical up...
  
  If you wanted to put in the actuators yourself, I'd say go for it. Give yourself a weekend with at least 12 hours of time to spend on it. It won't likely take that much, but you don't want to run out of time half-way through. Maybe you could get the actuators installed on one day and the brain on the next.
  
  This one seems like a good reference for the actuator install
  
  OR - you can buy the genuine mazda actuators and then you only have to worry about wiring them up. A quick google shows replacement parts available for $20-40 each door... You might even find the car/door is pre-wired for them.

You could make something like this https://www.youtube.com/watch?v=UMskp5M8Igw using a cheap car door lock actuator and keyless entry system

http://www.amazon.com/Remote-Central-Locking-Keyless-Controllers/dp/B006QH9C5A
http://www.amazon.com/Universal-Power-Actuator-12-Volt-Motor/dp/B00CZBQCR2/ref=pd_bxgy_e_img_y

You can use a small 12V motorcycle battery as the power source.

http://www.amazon.com/ExpertPower%C2%AE-Rechargeable-Sealed-Battery-BLMFM12_7/dp/B003S1RQ2S/ref=sr_1_2?ie=UTF8&qid=1415825130&sr=8-2&keywords=12v+battery

And if you feel ambitious you can hook it up to a solar panel recharge so you don't have to charge or swap the battery.
http://www.amazon.com/Instapark%C2%AE-Black-Mono-crystalline-Charge-Controller/dp/B004FOEUI0/ref=sr_1_3?ie=UTF8&qid=1415825203&sr=8-3&keywords=12v+solar+panel

This is the one I have. Works great. Removed the stock one and tapped into the stock keyless entry harness.

https://www.amazon.com/Docooler-Central-Locking-Keyless-Controllers/dp/B006QH9C5A
Been using this on my e30 for the past two years works great, relatively easy to install just follow the thread on r3v, the video from the reviews is also helpful