Finally got round to looking at this, my R plate PH1 doesnt have footwell lights, and I kinda like the effect.
The light units are the same as the ones in the doors (puddle lights)
Remove the glovebox and look above the heater blower, there are two multiplugs pointing down, the box these go into is N82 Integrated Services Unit. In the late PH1s and PH2 and PH3? this controls loads of things, including the interior light logic.
On mine the plug for the footwell light was tucked up inside the dash, the attached picture shows the lead in question. BLack plug with solid purple and red wires.
The big wire bundle it splits from comes from the top of N82 and goes off towards the door.
Job done.. Easy Peasy
Now onto the drivers side... Uhhhh
Try as I might I cannot find the plug, assuming its there.. The loom from the passenger side just seems to have a load of spliced in wires that connect to the meta alarm where I would expect the lead to split off from..
It may be tucked up above the fuse board,, but trying to find anything in that mess is a nightmare..
Plan B,, run a lead from the passenger side through to the drivers side, plug light unit into female spade connectors.
Finally my feet are bathed in the warm glow of a w5w bulb
Early PH1s have the interior lights controlled by the Ceiling light unit.
Late PH1s have the N82 ISU, and the diagrams in the manual show foot well lights. (though absent on mine)
PH2s have the same N82, but the circuit diagrams in the manual show no foot well lights.
If you can get access to the connector C of N82 (I suspect this is on top, not accessible without unbolting it!) then pin 1 (purple) is the switched ground for the interior lights.