Yeah, that's what I figure.

Is it common to have to rig up something like that to get an aftermarket radio to work?

__________________
'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09