Following is some information I found on this topic (unedited)

WRT the reverse warning system, I would recommend the Poron USA 3 sensor system http://www.poronusa.com with the beeping (inside the car) & visual indicator. If it's too expensive, the 2 sensor system with the voice distance indicator is also another less expensive option. FYI, the 3 sensor system can also accomodate a splitter/sensor, bringing it up to 4 sensors altogether.
If a several people (in this topic or in Townhall) would like to do a group purchase, I'm sure Poron could knock down the price. Autotoys.com has excellent DIY instructions + pictures of the 2 sensor system on an ML. I referred to extensively.
Picture of the Poron system installed on the ML:

This CBC Marketplace report
http://cbc.ca/consumers/market/files/cars/revsonar/index.html
was actually the main motivator for me to get the reverse warning system. Have a look at the video clip of the TV, if you have the time. It's quite interesting.

Also see http://www.autotoys.com

Hope this helps

...Tracy