OP, are you mechanically inclined at all to work on cars? If so, you should work on it yourself.

As others stated, it's highly unlikely the electrical panel failed.

About 15 years ago I visited an independent Porsche shop. A good one.

He had been working on a 928 with electrical problems for most of the day, and was excited to share his discoveries.

the panel (not sure which on, he said main panel) had a few connections where the wires were being pushed out of the panel when relays were plugged in. I guess the spade connections were no longer held tightly in the panel. The relays looked fully seated, but wires were pushed out.

he said only by unbolting the panel and looking at the back as a relay was plugged in did he notice anything.

but that was a master mechanic relaying his discoveries to a novice, so something may have been lost in the process.

No - neither '88 or '90 are compatible. Some years (rarely) share a panel with other specific years (e.g. 87/88) - But '89 does not.

I'd question why it needs to be replaced though - who determined this? The panel is modular - most things can be replaced individually. So unless the frame of the panel is actually fractured or damaged I don't see how any failure can't be repaired relatively easily.
What is the fault issue that you (he) are trying to resolve (you mentioned HVAC Blower?.. more? ) Note that many very competent mechanics are not actually experts at electrical debug (more so on a 928) - so bear that in mind. He may just need help, pointers, ideas...

Alan

I would swap ignition switch first-
also do a “Mr Merlin” aka “Dexoit-Man” fuse panel refurbishing.
Well documented on this forum.

I would swap ignition switch first-
also do a “Mr Merlin” aka “Dexoit-Man” fuse panel refurbishing.
Well documented on this forum.
Do it yourself!