General Landy 90 wiring 1989 2.5TD

Hi Folks,
Discovered something interesting tonight while attempting to fit a new gauge in place of the original battery, a) it doesn't fit, the new gauge that is and b) there's a black box fitted to the bulkhead above all the wiring. It's not connected but I found the plug, when I plugged it in all the headlights came on regardless of headlight switch. Any idea what this could be?
I'm also wondering why the trailer lamp flashes rather than the "turn" lamp. There is a clicking noise and that comes from a relay floating about next to the main fuses. There's also a knob on the top of this relay, I thought maybe it's the flash frequency, nope - turns out turning this effects the warning, in my case flashing trailer, from one extreme to the other results in either no trailer flash or it on continuously. Never come across one of these relays before. Of course, I set it to just right so it's in sync with the flashers. As for the new gauge, a GPS speedometer 52mm I'm going to have to fit that elsewhere.. Maybe where the ash tray sits. Any help, most appreciated..

The box you found is the voltage regulator for the dim-dip headlights, which turn on the headlights at reduced voltage when the engine is running and the sidelights are on. They tend to fail and give headlights all the time, which yours presumably has. They're no longer a legal requirement so the previous owner has diconnected the faulty unit.

The adjustable relay is usually fitted when LED lights are fitted. It's variable to prevent the double-speed flashing which results from using a standard flasher with LED lights. To set it up properly, turn the adjuster so that the trailer lamp only flashes when a trailer is plugged in. If you have no trailer connected and disconnect one of the car's indicators, the flash frequency should increase.

Because the turn lamp on the dashboard earths through the unused indicators (so indicating left it earths through the right) when you fit LED indicators there's too high a resistance for it to work correctly. The simplest solution is to revert back to standard indicators with bulbs.

As stated above, the knob on the relay is used to adjust the point at which the dash trailer light comes on due to trailer lighting being plugged in. If the turn lamp isn't coming on you need to check the bulb, or maybe replace the relay. BTW It's quite normal for the trailer light to come on once when the indicators are used without a trailer plugged in! (I believe this is a test function) The correct relay used with Wipac LED light units works perfectly.