Outer Limits 4x4 Board

I have bought myself a Twine and need a little help.

Its says in the installation instructions "Identify the cabin heating line"

Now I Have a 99 4.2d so i have figured out it is one of the two hoses rubber right at the back of the engine that strangly enough disapear into the cabin.

How do i figure out which one is the "supply line" as that is the one I need to tap onto.

Also do i need to drain all my engine coolant or can i get away with just undoing one of the top hoses during installation and then just do a top up later.

Thanks for any help

From memory the feed line should be the one coming out near/ on top/ on the side of the thermostat housing?
I think?
from memory?
maybe?
sorry

inlet is top of thermostat, no need to drain sys, just tap into the line with a tee, you'll lose a bit of fluid but run the motor and top it up.

Thanks for your help

I shall give it a go on the weekend

Thanks again

Just pull one of the hoses off and start the engine, youll see which way the coolant is flowing.