Hi,
I'd expect this approach to have some disadvantages, mostly that the subwoofer only works for BlueSaab output, not when playing radio or other IHU source.
Also, the output from the RN52 module is not line-level, and it's balanced. You'd still need some amp and other circuitry to deliver it to the sub amp in the most popular format. At that point, you might as well use a more conventional after market sub-amp driver approach from the existing IHU, or, depending on your IHU, there is an output for amplifiers (e.g. AS3 on the 9-3 has a separate amp for the door speakers (woofers). It is a proprietary plug but you could probably get the required cable for splicing from an auto recycler.
Sorry to be a downer.
Cheers,
Sam.
9³ 5D MY02 - Stålgrå, AS3; iOS 16.1; BlueSaab v5.0-p1+Amp v1.1, SAAB-CDC v4.1 with mods