Light stays on red.
What steps to troubleshoot?
No juice going to handbrake sensor or fluid bottles per testing.
What’s the next step?
Red wire in pic above not plugged into anything. Could this be our culprit? Diagram shows it goes to fuse #6.
From memory, both the switch on the handbrake and the float switch in the reservoir make an earth contact when closed. So you won’t get any voltage off the wire as it is not powered.
So if the light is staying on when you have both switches disconnected there must be a short to earth somewhere.
PS I like John’s sunken switch idea. Try disconnecting the RG wire from the bottle and see if it goes out.
If that’s not it try disconnecting the RG from the handbrake switch.
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Typically it can be one of two things.
First a saturated brake fluid float which sinks low enough in the brake fluid to activate the light. Easy to check. I remove the cap and float, being careful not to fling brake fluid everywhere, and turn it upside down. If the brake light goes out the float is bad.
EDIT: You can probably achieve the same thing by just pulling the red/green wire from each cap and observing the light.
SECOND EDIT: I see Andrew already suggested removing the R/G wires.
Second is the switch at the base of the handbrake lever. They get out of adjustment (wear, bad Karma, who knows why). Adjustment entails removing the console and driver’s seat.
Hope for option 1.
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This wire is not the culprit. It appears to be the Red/Black “mystery wire” which has been discussed here before. The wire is only connected for German market cars, and bypasses Fuse #5 (not #6 - no red wires on Fuse #6, only White (input) and Green (fused output)) for the circuit to the tail lights. I guess the logic was that it’s not OK to have the tail lights go out due to Fuse#5 blowing for some other less safety critical reason.
If your Handbrake/Brake Fluid level warning light is always on (with the ignition ON), then something is grounding the circuit. This could be (a) the fluid level switch in either brake reservoir - eliminate that cause by disconnecting the two brake reservoir switches in turn, (b) the handbrake switch - eliminate that by removing the seats and console to get at the switch, or (c) a short to ground in one of the wires. If you ferret around under the left side of the dash you should find a 4-way connector with 3 Red/Green wires going into it. One will go to the warning lamp, one to the two brake reservoirs, and one to the handbrake switch. If you run into problems determining the source of the fault, access to this connector may help with the troubleshooting.
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Thankfully interior and seats are not in so that’s helpful! Will trouble shoot that now!
Very helpful! And I now remember asking about the mystery red wire a couple of years ago…. I’ve slept since then.
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Ok.
We unplugged the red green from the handbrake. Light stayed on.
We unplugged the red/green from the bottles and light stays on.
So am I understanding grounding seems to be the issue then?
I assume one of those sentences actually refers to the brake bottles. Unplug the R/G from each bottle at the same time, just in case both floats are closing.
My bad. Yes. Disconnected both red green from the bottles and light stayed on.
Grounding issue!
Handbrake switch is good.
So we still can’t find issue. 
Time to go and find the connector I referred to under the left side of the dash. It is probably close to the handbrake/brake fluid warning light. With any luck, the red/green wire that goes to the warning light doesn’t disappear into a loom, but is just a single wire and you will be able to see it. If so, just disconnect that wire from the connector and see if the light goes out. If it goes out, one of the other two green/red wires (one for the fluid reservoirs and one for the handbrake switch) is grounding, and you can try to identify which. If the light still doesn’t go out (I suspect this may be the case), then something may be grounding the warning lamp socket causing the light to be on all the time.
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If that is the case you have a short to earth somewhere in the circuit past the bulb.
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Thanks guys, that the best I can figure. Now to try to determine where the grounding is occurring.
You are correct, it enters the loom.
Using the wiring diagram in this post, looks like if you could locate the connector where the wire from one side of the warning light goes, you should see three wires at that connector. One side has two wires the other side has one. Remove the single wire and if the light goes out the problem is in the fluid warning circuit. If it stays on the the problem is in the handbrake circuit.
The only way the light goes off is if we disconnect the wire to the warning light.
Disconnecting the bottle RG wire, nor the handbrake wire results in the light turning off.
Following the wire is difficult as it is goes directly into a black loom.
Trying to get this resolved BEFORE the interior goes in. 