A member on cadillacforums said if you use this you will not need to relocate the factory HU. Everything will work fine. IDK if I should trust this. You guys are much more knowledgeable.
Another member on there was said, "Most of the problems are with Eldorado and Deville (not that ’98 and up Seville is further). The problem is mostly with the software and appears to be random occurrence (some cars have-it,some don’t) probably related with the addressing on the CAN bus."
The deal is that the shop I go to said they can get me the dash kit for $40 and a wiring harness for $60, so $100, and then charge me $100 labor to install the HU and relocate the Bose. This option would have me spend $120 then $40 on the dash kit then get charged for labor. So probably the same price if not more.
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1999 Cadillac Seville STS 2 12" JL W3v3 in a HO Box JL 500/1v2