Tuneable FM transmitter

I'm going to get a GPS for my laptop. The software has Voice directions, but the laptop's volume is to low for me to hear using the laptop's speakers while driving. ( Cargo van ).
I seen gismos like, "http://www.walmart.com/catalog/product.do?product_id=4694330" , but I would like something that I can "over_ride/power" so that it will come through the current station in which I am listening too.
Is there an adjustable FM/transmitter or anyway to adjust a prebuilt unit (fm/transmitter)?

You might simply use a relay to disconnect the exterior antenna while the GPS/laptop is transmitting - and keep the transmitter off when there is no voice instruction.

You might find that by placing the transmitter in close proximity to the exterior antenna that there is enough power to do that already - just operate the transmitter when voice instructions are being rec'd. You might also operate the transmitter constantly - into a dummy load while you don't want to hear it - then into a simple antenna when you do.