Having just spent yesterday ripping out some very hacked together wiring a 'professional car audio installer' did on my 986 about 10 years ago, I urge you to consider tackling this job yourself.
My '02 has the Bose upgrade and did not require any interface module. It's powering the Bose amp from speaker outputs on the head unit + a remote turn on trigger.
I installed a Pioneer AppRadio 4. It's not the newest tech, but I found it on sale and it suits my needs just fine (FM radio + Apple CarPlay).
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'02 Boxster S
'16 GTI PP
'17 Bolt (with a B)
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