I don't have a beautiful classic XJ unfortunately. I do however have it's close cousin, a Triumph Stag; launched soon after the Series I XJ in 1970.
The Triumph Stag's AC system I strongly suspect was very closely based on the XJ's design.
Unfortunately for Australian delivered Stag's, they never officially came in with the excellent factory integrated AC. Instead the local 'Leyland Australia' group would just sublet the work out to specialists for anyone that wanted a hidden away air conditioner.
At least someone locally did come up with a reasonable 'hybrid' design. This used the lower half of the Triumph Stag's 'Smiths' branded heater assembly and completely did away with the upper half, including ditching the Smiths small blower fan.
It was also neatly wired up to make use of the standard factory 2 speed fan/blower switch which powering a 2 speed (Japanese sourced) blower fan behind what looks like a Jaguar XJ6 evaporator as well.
So at first glance of the interior of the car, you couldn't easily tell if anything was 'added on' unless of course you noticed the thermostat; which naturally had to hang out somewhere.

Reading the 'Air Conditioning' section of the Stag's owners manual, it reads "selecting the cooling system will automatically bring in the operation of the blower units [there were actually 2 separate blower fans] at low speed. From this condition high speed may be obtained by pulling the switch knob fully out".
Got to love the 1970 wording, where in most markets air conditioning was still a (expensive) and rare luxury.
What the owners manuals' really saying is that when you slide the top temperature lever under the ashtray to the left the interior blower/fan automatically engages in it's first (low) speed without actually having to pull out the fan switch, which still remains in it's 'zero' position.
A nicely engineered touch. If you wanted the second higher speed you just pulled out the blower switch from its zero to position number 2.
Like the factory integrated AC system, I wanted my Australian hybrid systems' Japanese blower fan to turn on to the low speed position as well when I "select the cooling system"; in my case 'selecting the cooling system' means when I turn the thermostat knob on.
My electrical engineering knowledge is mediocre but I think I managed to get it to work this way with a couple of relays linked to the factory blower/fan switch and thermostat - see attachment.
Not sure if this part was ideal, but I used the cars factory installed 'window lift circuit breaker' to power it up.

I'm now stuck on how to wire in a 'Dual circuit, 4 wire Low/Medium/High (trinary switch)...
A Binary switch would've been a simple wiring installation but I wanted the trinary switch's ability to turn off the dual condenser fans at high airflow speeds when not needed. With the "blanketing" effect on airflow that condenser fans can have on the main engine fan's.
I've reached out to the Triumph Stag Forum. It's UK based so not much AC interest.
Any help would be appreciated.

 

If what you are trying to achieve is to only use the electric fans on the condenser when required, then you could copy the way the XJ6 S3 has it, which is via a thermostatic switch in the radiator. Once the coolant temperature passes a certain threshold, the fan comes on, and stays on until the temperature drops back below again. This is latched by a relay so that the fan will stay blowing after the engine has been turned off until it cools below the threshold.