Radio fm modulator

Please read, I really need help. I bought a wired fm modulator to hook up in my car stero but on the modulator there is a black ground wire which i might just connect to anything metal and a red wire that I need to connect to get power for the modulator. I figure out that for my car the power cable for the radio is white/red so how can I connect
The modulator cable to the white/red cable for a solid power source? Should I cut the white/red wire and then take the modulator wire and solder, use a push in connector, or a wire nut?

Definitely solder the connection, unless you're planning on getting rid of the car soon. Instead of actually cutting the wire, try stripping back the insulation more in the center of the wire.
http://www.service.kleintools.com/Marketing/Catalog_Imagery/11063_ICON.JPG
With that ^ one side grabs the wire when you squeeze, while the other side cuts the insulation and pulls back one side from the other as you squeeze. You should be able to find them at any home improvement store or auto parts store.