Viper Car Alarm. Interface Module what?
 

So I posted a while back and got my hands on a Viper Car Alarm 5706v. Anyways.. I found a legit installer thats charging $150. No biggie. Then he wants to charge me another $100 for interface module. Ok what?.

This is where I need your guy's expertise. I know some cars such as ours, require a interface module to proxy the key for the new car alarm devices. What is the price I should be paying for the interface? Will any work? Can I just buy one off amazon and it will work? I'm scared of buying something and its not compatible with the car. If anyone installed an aftermarket alarm, please let me know what you did when it came to the interface module.

Thanks.