There is a power distribution module next to where the ECU is inserted. From what I know there are some non-serviceable fuses inside of the module. It looks like this:
It's definitely a bit tricky, if you get one side out first the other is significantly more difficult. The best way I can describe it is to try and keep the airbag as level as possible while getting the last side out. It just takes a lot of persistence
I noticed that StacyOz's account was deleted, along with all their posts. They created a useful guide on using Bimmercode with the Supra. I'm reposting it so others may continue to reference it. Hopefully they don't mind that I do so.
By the way, a tip for anyone who's installed a MMI box and use it through the AUX input.
AUX has its own volume slider separate from your main volume setting. If you tap AUX in the iDrive menu you'll see an icon on the left which lets you adjust the AUX volume. I was running on minimum volume...