Location of T5 Alternator?

[Transporter]

The battery is storage of the energy, like capacitor.

What you're observing is a battery voltage drop. When battery voltage goes down because you put more load on it, at that moment more current is flowing trough the wires to the lights. Upgrading battery negative lead could help.

It would be ideal to upgrade all 3 but you may try positive and negative from battery first if you can't get to the alternator wire. Then make sure that the battery is big enough.

EDIT:
I have just checked the specs of that Optima D34 battery it is 55Amp/hour battery, physicaly too small, I wouldn't trust it that it would always start my T5.
I have 80A/h battery in mine, that is standard which came with T5.
Optima YTS 5.5 would be more suited OPTIMA
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[lemmiwinks]

At the moment I think I'll try and upgrade the 3 wires firstly.

I've read that the Optima D34 battery can pump out power very quickly and recharge quicker than a standard battery. I'm not too sure what the a/h rating is on the standard battery will have a look tomorrow.