Help needed - DAB aerial in a fibreglass car

[Cooper1999]

Hoping someone on here can help (as I'm not great with electrics!).

I'm in the later stages of building a GRP kit car (a Mini Marcos). I'd like to fit a DAB head unit ultimately and recently purchased an Autoleads DAB-AA1 aerial unit.

Having read the instructions I'll need to attach the aerial to a rear, side window because both the front and rear screens have heating elements. But one line in the instructions is confusing me. After instructing to remove the sticky pad bases and stick to the window, it states 'Stick the earth plane strip to the metal surface'.
Now, the antenna piece (that sticks to the window) has a short 'L' shaped piece with a sticky backing (it also has a longer, thinner 'L' shaped bit that looks to project further into the window area). Is this short L item meant to stick on to i.e. the metal part of an A pillar? In which case - if this piece does need to be attached to metal (for earthing presumably) am I stuffed?

Sorry if this is a dumb question - never fitted a DAB radio/head unit before.

Thanks in advance.

[Cooper1999]

Thanks broncoupe.
There is an battery cable earth return quite close - would that work?
Cheers,