Relay connections for Heated Windscreen

Hi,
I've just installed a heated windscreen on my TD5, on Sunday I intend to do the wiring side of things. The Britpart wiring loom has arrived today, the instructions suggest connecting the coil side of the relay to a switched live. Is it not best to connect it to an engine live to prevent battery drain when the engine is not running. If so, what would you connect it to...........any advice appreciated ?
Finally, the windscreen has two electrical connectors (bottom left and right) - does it matter which way around the + and - are wired ?
Thanks.

I use a voltage sensing relay to provide a feed for the switch so its only live at 13v or above
The polarity of the connections does not matter, I would suggest the positive connection is as close to the relay as possible which in turn will be determined on the placement of your control relay

mine comes off my alternator.

Another option is to use the Oil pressure switch to provide a neg when up to pressure (engine running)

Thanks for the suggestions.
On a separate note - the heat windscreen is now working well however, the instructions said to wire the relay direct to the battery with an in-line fuse which temporarily I've done. I would like eventually to wire it straight into the main fuse board, I've checked and I have a spare 30a fuse for the air conditioning thats not installed. I don't really want to use 'breakout wire' fuses as they are bloody expensive, what's the best way to connect to the air conditioning fuse ??
Sorry this is all new to me !

duplicate thread https://www.landyzone.co.uk/land-rover/heated-windscreen.351188/

Further to my last post I'm experiencing issues with my new heated windscreen set up now.
A Fuyao heated screen was fitted to my TD5 and I used a Britpart wiring loom, everything appeared to be ok however, I'm concerned as the fuse is getting really hot. I've since tested the current that the screen is using and its 24A, the inline fuse supplied with the Britpart wiring loom is only 20A. Have I purchased a wiring loom that is not man enough for that screen ?

Any suggestions...........as it stands the screen is completely isolated ?

Sounds like the harness may be under specified. What was it rated at ? is it on any documentation or listed on the web site ?
For 25A current draw I would use wires rated for 35A (4.5mm std or 3mm thin wall) and a fuse of current draw plus 25% i.e. 30- 35A fuse
Check your entire wiring supply circuit can carry the drawn current and the fusing protects the smallest rated cable in the circuit

normally 4.0 mm cable from relay to screen 6.0 mm from 30 amp fuse supplying the relay on the 2007 puma reduced to 2.5 mm cable on 30 amp fuse so must be relevant to element wattage of later screens