Note that I am newer than a Supra newbie. Never owned one, test driven one once. Just been doing a ton of research (25+ hours total I'd guess in the last two weeks.) But I found some info that might help, but I can't make sense of it...
http://www.cygnusx1.net/Supra/Librar...px?S=Main&P=24
look at c9-c12, which is for the combination guages. It might tell you what guage specifically is on what fuse/ground wire.
here:
http://www.cygnusx1.net/Supra/Librar...px?S=Main&P=28
It looks like the line that runs to the combination gauges splits and uses two separate connectors to tie back into the main harness?, if you look at e5 and e4. That could explain why only the fuel gauge and possibly the check engine light do not work but the others do...
Also, you might include the year and is it a 7m-ge or 7mgte, automatic or manual. Not sure what of that info is needed, but easy to include it in case it matters. You may also want to include what exactly you've changed, as anyone who actually has had their hands inside the dash may have a eureka moment when they hear you list the things you've done.
Anyway, hope that helps, you should look around that website if you haven't encountered it before, it's a virtual treasure trove of information. Sorry I couldn't help more =\