all right, gosh dang it!... I've done this a few times before. It doesn't get easier either. I'm not Real Time Engineering and I'm not responsible for any of their products or customers ( alltough I did jump on their BS train and ride it for bout 8 years). Anyway,
Most owners have a basic plug in type voltage limiter, a 2 post fuel gauge, and a 2 post temperature gauge, and a oil warning lamp. Then there are the variations. Some with a 3 post fuel or temp gauge that has the voltage limiter inside it, whether they have a oil gauge or basic oil warning lamp. The most important point is... a basic 2 post gauge is not chassis grounded, whether is fuel, temp, or oil. The 3 post gauge with limiter inside most be chassis grounded for that internal limiter to work. So now we want to install and use a external regulator like all those with all 2 post gauges. No problem. Too simple.
Lift the 3 post gauge from the housing and turn it over. A piece of metal on its backside grounded the internal limiter to the housing. You'll also see a unpainted spot in the housing floor. Cover that piece of metal with a layer or 2 of electric tape. It is now isolated from chassis ground just like any other 2 post gauge. If you want to go to the added effort to lift a temp gauge or oil gauge you will find it is not chassis grounded. Only current path is from post to post.
Next point is the 12 volt supply. Where should it go? It originally went to the limiter wherever the dang thing was located. If you're adding a external regulator anywhere in or behind the dash( it dont have to be on the back of the inst' panel if there isn't a good place for it), take the switched 12 volt wire, be it black or blue with white tracer out of the OEM harness connector and route it directly to your substitute regulator (no matter who you bought the feckin' thing from !) . See here's the thing, I suspect that RTE were either ignorant or wanted owners to open their 3 post gauge so they could see there is more wrong than the limiter alone. I did know well before I ever found this forum that RTE were E-body people. At that time they didn't have a clue about the 3 post gauges and wouldn't touch them. That was about 10 years ago. Not only did my brother and I figure out what they didn't know, we also discovered a possible scam, or way to generate more income. I decided that tehy had only ran with what a guy named Ron Erinberg had published on a Imperial website. Yeah I found a way to profit a little from it. I am a member of the so sue me tribe LOL.
Next point and why it took me soooo long to respond here... I tried to tell another member that he only needed to substitute gauge voltage source and really didn't need to open his fuel gauge. Then he mixed what I had told him with what he saw at the RTE website. That chit didn't work. My bi polar disorder clicked in and I dang near called him a idiot. I still feel bad about it. I truly only wanted to help. Bottom line, if you're willing to Completely isolate that OEM limiter, there isn't need to open the gauge. Take 12 volts away from it, take ground path away from it, take 12 volts away from it, take chassis ground away from it, it may as well be rolling around in the glove box. The 5 volt supply to the gauges goes from regulator to gauges. If you early instrument panel has a weird metal clip where you need to attach the 5 volt wire, that dont seem doable. So, route it to another gauge (like to the temp gauge as shown above). The printed circuit board will conduct it on to the proper posts.
I'm sorry that this post has ran on soooooo long but you haven't heard the worst of it yet... Recall when I mentioned the little piece of metal that provides a chassis ground for a internal limiter? The 3 mounting studs are swedged into a fibrous board on the back of a gauge. That board, along with weight of the gauge, distorts over time. The speedy nuts used to mount these gauges get loose, add a little corrosion, the chassis ground path is lost, the limiter stops functioning, or comes and goes with bumps in the road. Some number of 2nd generation rally gauge owners shipped there 3 post rally fuel gauges to me, with nothing wrong with them, years before you got here. Those owners only needed to do a little cleaning and tighten some nuts to get chit working again. In most cases it likely wasn't going to continue working for years to come, so I went ahead ad totally amputated their OEM limiter and installed and/or provided a substitute solid state regulator.
One more point that you dont have to read... Several members here have created what is called "sticky" about how to fix these old cars. That aint me. I really dot give a chit anymore. Blame it on too much water under the bridge, too much BS from other vendors. The post you just read is out there. They can do what they want with it. Like I said many times before, I didn't start this feckin' mess and it aint my lifes ambition to clean it up.
