I realize this is an older thread.... but in my 89 I added the tach cluster (I never understand why automatics tend to have a tach, but not manual transmission vehicles) . I did not research anything about the swap, I just did it... ran into the cable issue, so I moved that over to the left, which resolved that problem. But I realized that my clock was not working. (the old unit had no clock, so I was not looking for the clock.
well I finally got around to getting that working too. and as mentioned you have to add power, a constant power so it holds the time, unless you want it to start from 1:00 every time you turn the key back on.
so here is what I did.
The red circle represents the 2 contacts that are added. The pink circle is the one that needs to be hooked to constant power. (you could stop there if you want, your clock will work fine).
the other contact in the red circle is what dims the clock at night when you turn on the headlights. I just soldered it to a connection on the circuit board, there was already something soldered to that spot, so I just added this wire to it.
the white circle represents the spot it was soldered to. this way there is only 1 wire that is routed off the cluster, the one that goes to the constant power.
could I have left it alone and not had the clock working? well most people would be ok with it, but small things that dont work bother me, so I HAVE to get them working !
Other than the clock, and moving the speedometer cable over to the left everything was plug and play.