Help! elecrics go funny when lights are on RT104

[Adrogen]

Hi everyone need some help,
RT104 corona
when you turn the lights on (from parker right through to hi-beam) the dash electrics go haywire heat gage goes right to top, both blinker indercaters come on dim, dash lights dim right down and hi beam light comes on dimly when on low beam and then turns off when you turn hi beam on.

lights appear to work fine from the outside, not sure whats going on!

I will mention that I have just replaced the front parker globes with 10w bulbs when previously they had 5w. The staff at Repco said higher wattage shouldn't cuase any issues.
please help, car is going in for rego very soon
Thankyou, Adrogen.

[MWP]

First thing to always check is ground points under the dash.

Got a multimeter?