I remember seeing an auction on Ebay about 2 years ago for an aftermarket credit card sized transponder that you kept in your wallet. As you approached your car, the transponder sent a signal to the car (which had hardware you needed to install) and unlocked the doors for you.
It was Taiwanese, and I think the name was Xandoo, but a search showed nothing of interest.
Does anyone know of this or a similar product?
What you are describing (In my mind) is a Proximity card. WHich is used a great deal in Access control systems. WHile I cannot off the top of my head name a company that produces, I know it is just a matter of having a prox reader wired to the door lock cynoid (how is that spelled?) Which would be easy enough.
oh check here one of these companies might be the manufacturer of what your looking for.
http://www.manufacture.com.tw/tpo4/product-category/products/z409002e.htm
Solenoid? 
Sounds cool, I’d be really interested in finding one of these things. Does it automatically lock too? 