Based on the code used on the key fobs from this site: http://store.diyembedded.com/index.php?main_page=product_info&products_id=31

I've compiled the code, sent it to the modules and voilá! I can decode the keys sent.

The keys are emulated by the Pi's GPIO's. I've used one pin as output for the transmiter and one as input for the receiver. This way I can change the state of the tx pin and see if the rx pin changed as well. This way I can check remotely if the code and the modules are running as planned.

Also, I'm sending the battery voltage on the data frame. This way I can get a warning from the car when the voltage gets too low.