Sears sells a replacement garage door openers that about the size of a matchbook, and about 1/2" thick.

I put it between the seat and the console on the little horizontal trim that runs front to back. Very inconspicuous and your hand just falls to it. I secured it with a little pad of the "hooked" side of Velcro.

Works great!

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)