No power to heater display has me stumped
 

Yesterday my heater display was blank when I started the car (2000 base 5-speed).
Last thing I worked on was replacing the brake booster. When completed, the day before yesterday I went for a short test drive and did not notice if the heater display was working... it was working before the booster change. I don't see what the one could have to do with the other, and I am not sure of the timing of the problem.
What I have done so far...
Checked the relevant fuses, switched other relays for the relays that are involved, #9 and #18.
I pulled the display/control unit out of the dash and checked that there was 12V on pin #9 and ground on pin #15 of connector D. For me this tends to confirm that the fuses and relays are OK.
Since I have another 2000 Boxster hanging around in the driveway I pulled its display/control unit and plugged in the supposedly problematic one and it worked fine in the other car.
So where do I go from here?

It was not clear in the original post that it is not only the display, no display and the buttons do nothing.
Thinking about it I decided that this problem, which occurred just after the brake booster change, might have been caused while I was making that repair. I went back to the frunk and inspected everything. I see no indication of an anomalie there and, honestly, none of the wiring is near where I was working or a place where a wire could get pinched or cut.
I am going to retest the fuses and relays that power the HVAC controller, but the fact that I have 12V and ground at the proper places on the connector would seem to say that the input power is OK.
I am wondering if there is like a signal voltage that activates the controller. There are so many elements that feed to or from the four connectors on the back of the controller that I have no idea where to look.

I found the problem! A bad fuse... E7. I had already checked it and it looked fine and passed the continuity test. When I rechecked it I noticed that the contacts were a bit crooked so I switched in another and voila the HVAC works. So it was not related to the brake booster replacement. Coincidences do happen.