Hino number plate light wiring - wtF?

Getting our 2005 Hino GH1J lazy axle ready for the RTA HVI on monday. Number plate lights weren't working, bulbs were good. Couldn't find any damaged wiring, cleaned plugs. Getting 8 volts at the socket. Tail lights working (in fact every other light on the vehicle).

Wires go into the loom behind the cross member, so I;m thinking something's not right in there. I pull it apart, and find the earth is good, and the hot wires are together with one (of two, other is the same feed as the reverse light) hot wires for the reverse buzzer. Not sure why.

I put it in reverse and turn the igntion on, buzzer working, 24v at the light sockets, but no lights. I know, not possible. But it gets better, I remove the bulb, buzzer stops working, voltage back to 8v.

So I took the easy way out - they never seem to be interested in the buzzer so I cut the feed and lifted the hot from the tail lights. They now work but no buzzer of course.

So can anyone explain to me how the number plate light and buzzer wiring is meant to work?

You need a positive and a negative.
Check the earth.

You need a positive and a negative.
Check the earth.

Earth was good - an earth wire coming out of the loom. I did check it for continuity with the chassis. They work now they get the hot from the tail lights

It does sound suspiciously like you have power on the earth lead. Getting only 8 volts on the hot side sounds like a short that is letting some power across to the earth. I guess its similar to a charge light on the dash, it goes out when the charge is equal on both sides of the lamp. Checking the earth wire will still give continuity to the chassis, even though it may be live.

When check for continuity using a multimeter will show there is a circuit but wouldn't show up resistance that may causing voltage drop

It will if you check the continuity by checking the resistance. I always do it this way as I have seen many people fall into that trap.

Correct

Yeah I check resistance, although I normally use the alarm which comes on when resistance is basically 0. But I couldn't find that multimeter so was using my analgogue one and it had 0 ohms.

Power on earth sounds like it could be right, I'm still suspicious of the reverse buzzer. Obviously this method worked as it's all nice Hino wiring and worked last year... But why on earth would you wire the buzzer with the same +ve as the number plate lights?

I dunno what the answer is here but be savvy when using digital meters on truck wiring. They are so sensitive they can sometimes show there is a voltage there but once any load is applied it disappears.

I dunno what the answer is here but be savvy when using digital meters on truck wiring. They are so sensitive they can sometimes show there is a voltage there but once any load is applied it disappears.

That must be what I experienced when I had 24v at the bulb holder (only with ignition on and in reverse) but nothing when I put a bulb in