which switch ,and the relay in your pict is wiper relay. any images of switch in question and image of the headlight connections

what year ,does it have the dim/dip relay and has that been linked out by the PO

may help

first search https://www.partsgateway.co.uk/parts/detail/td5-r380-gearbox-tunnel-land-rover-defender-90-gear-lever-housing-cover/41490856

check the header's for corrosion whilst the dash is off they are a two part male and female connector

Dale Radford of Radford bulkheads has done some nice engine replacements in the series one club back in 2011 but you would have to become a member to excess them .pictures belong to dale attached

you can get the original radiator re-cored from 3 t0 4 segment to increase cooling

take a gander the magic 8 https://www.hagerty.co.uk/articles/maintenance-and-gear/dvlas-dos-and-donts-the-risks-of-modifying-your-classic-car/#:~:text=Original%20suspension%2C%20axles%2C%20transmission%20and,have%20a%20Q%2Dplate%20fitted.

you could fit a modern 3 way cable operated valve that shut the flow off to the heater and allows it to circulate back through the flow and return pipes...

was the original coil you changed 6 volts if it was 12volt then no ballast resistor fitted unless the original was a engine V8.any images of the wire and coil in question

the ballast resistor usually has a white purple from starter motor which supplied 12volt to the 6volt coil after the ballast resistor on engine cranking and a ignition (white) feed through the ballast resistor to reduce the 12volt to 6 volt when engine running so the wire should run from the...

https://www.rangerovers.net/forums/range-rover-sport-l320.9/page-79 last but one item

ifo taken from Hella catalogue ,your starter has a pre-engage solenoid so should not draw a large current have you tried attaching separate earth wire to the relay to eliminate the alarm ECU the B/O wire when it's a none starter 4-PIN SOLID STATE RELAY Normally Open Relay (Solid State /...

think it's related

I would lose the post code are the battery's fully charged, have you checked the battery earth connection on the main battery

no water in trailer socket or caravan plug check link 5 in engine fuse box as this feeds both seats and fuse 34 to items in the body control module .Also check engine fuse box for corrosion or water ingress if seat power is a intermittent loss

Colour

any wet carpet events ,sun roof drain heater matrix leaks check header 289 ,it's a two part header an can be separated to check for good connections and allows you to check each indicator in the circuit and the two relays

see

check headers