97 Cadillac Seville the guy you bought from said he used to listen his iPod in car where is connection at car?

A 1997 Cadillac Seville wouldn't have a dedicated iPod connection. iPod interfaces as we know them today didn't exist back then. The seller was likely using an auxiliary input device, or possibly a cassette adapter, to play their iPod through the car's audio system.

To find out how he connected it, you should check for:

* Cassette adapter: Look for a cassette tape that has a wired jack for an MP3 player. This is the most likely method.

* Auxiliary input (unlikely): Some higher-end 1997 models *might* have had an auxiliary input, but it was not standard. This would be a small jack (3.5mm or perhaps RCA) usually located in the glove box, center console, or near the radio. It's very uncommon for that year.

* Aftermarket installation: He might have had an aftermarket car stereo installed that included an auxiliary input or other connections for MP3 players. Inspect the stereo itself for any signs of non-factory equipment.

Without knowing more about the car's stereo system it's difficult to give a definitive location. The cassette adapter is the most probable solution.