I've picked a battery gauge from a 740 turbo (old style), and I'm trying to put it on my 1989 740SE. However, behind the dashboard the flexible PCB thing is different.
Differences I have noticed between dashboards which might help:
- Mine has orange needles, the dials are red.
- Mine does have two connectors in the middle (see pics) but when I tested these with a multimeter I got like 0.13 volts and yes it was set to the correct setting.
- The battery gauge has been tested and it works.
- Everything works on my dashboard.

Maybe some volvo electronics guy could tell me where I need to connect it up to (or give me some wires to splice, I don't fancy finding those myself)
These photos go from left to right