Sounds like you have the wrong switch.
300Tdi and early TD5 Defenders have the 'one touch' fog lamp ECU (as do all Defenders up until the end of production).
These should have a non-latching, momentary rocker switch.
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The Hella lamps in your mocked-up picture, are larger than Comet 500's
The Comet 500 is 163mm in diameter and is for vertical mounting only
I've got 4 x Comet 500's on my 110's ARB bumper -
A more suitable Hella lamp would be the Rallye 3000 -
Which can be mounted vertical or...
By jumpering across pins 5 and 8 like that, they've by-passed the hazard switch's internal circuit that powers the indicator flasher relay, when the ignition is on and the hazard switch is off.
That would suggest that the hazard switch is faulty.
I can supply the 8-way Lucas-Rists connector for the hazard switch.
The connector housing (13H9237), supplied with 8 x terminals and a cavity blank pre-installed in cavity 6, they are £9.95 + P&P.
The terminals in the TD5 speedo's pink connectors are 'Junior Timer' terminals.
Available from all of the usual automotive electrical sellers, including me 😉
If that was a dig at my eBay listing -
LAND ROVER - BLUE 20-Way Header / Splice Connector & Terminals - YQC10005
Then, to buy the same from automotiveconnectors.com, with carriage and VAT, will cost you slightly more than from me.
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AFAIK, they're only available as part of the Genuine (Dixon-Bate) towing kit.
See - https://www.lrukforums.com/threads/tow-bar-electrics-socket-plate-part-number.105408/
The similar VVP100020 might still be available -
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It's a VOLTMETER - regardless of how it's described by the parts book and most suppliers.
If nothing else, the terminals on its rear give away its function - it has a 6.3mm tab for its single 'live' connection.
An Ammeter (apart from having a different scale, and AMPS on its face) would have...
From the chassis harness part number you've quoted (YMD111220), your Defender is an early TD5 (1999 - 2001).
I can supply some of the connectors you'll need for the under-bonnet connections of the chassis harness.
I have the 6-way Sumitomo male connectors - but, only in grey. The grey and...
I've edited my post above, to update the links to these files.
The wire colour codes shown, are as follows -
CODE / COLOUR
B = BLACK
G = GREEN
K = PINK
LG = LIGHT GREEN
N = BROWN
O = ORANGE
P = PURPLE
R = RED
S = SLATE (Grey)
U = BLUE
W = WHITE
Y = YELLOW
e.g.
WO = White/Orange...