Issue with starting car

[Haaz]

Hi guys, so my issue seems to be that starting the car sometimes takes a good 10 seconds for it it to crank and runs sluggish for a couple seconds and sometimes it starts perfect.
I’ve noticed that is the glow plug light stays on for a few seconds the car starts perfect but if the light only comes on for the needle sweep and back off it struggles, even after the car only sitting for maybe 2-3 hours.

I’ve had the glow plugs all changed, battery is all fine & changed the fuel filter in process.
Could my next thing be the glow plug relay ? As it seems an issue around the glow plugs not staying on when car hasn’t been started in so many hours.

hopefully someone may be able to help

pic of my car below

 

[Haaz]

If anyone has and idea where glow plug relay is located on this that would be helpful to,
Seat Leon supercopa mk2 is 2.0 TDI

 

[R3k1355]

The wiring harness needs checking for corrosion and stuff to I seem to remember.

 

[Haaz]

The wiring harness needs checking for corrosion and stuff to I seem to remember.

Is that the whole wiring harness? Or a a certain spot in it to check. I’m not the best with this stuff as never really touched under the hood before but know basics

 

[R3k1355]

Just check over the stuff for the glowies, any signs of damage to the wire or corrosion in the connectors