The overdrive solenoid in the AW70 is external and mounted on the left side directly above the pan
two M6 bolts retain it and the wiring loom to the switch needs disconnecting
you will need two new sealing rings (if the solenoid doesn’t come with them, I have them)
very simple to replace but do make sure to thoroughly clean the side where the solenoid fits as you don’t want foreign objects falling in the hole
as for the slippage in reverse, that could be the worn clutch plates, which usually equates to an overhaul. I still believe there is a chance your fluid level is low, so you might want to add around half a litre of fluid and try it again
let me know if you need the o-rings or any further assistance, can call if that makes explaining easier
