Slave Cassette Units

I have an '01 Silverado with an AM/FM single CD player in the dash. I would like to add a slave cassette unit to the normal area just above the console so I can play an MP3 player through it. I've seen a number of these units on ebay and elsewhere from all kinds of GM trucks from '95 on up. They all look the same but are all the mounts the same? Any info would be greatly appreciated!

I'm new to this forum, but so far the info on it has been fantastic!Thanks!