Check the voltage at the bulb terminals. Then break a connection and put in an ammeter.
You can piddle around with different bullbs all you want but there is no defying Ohm Law. Too small a wire is high resistance which is voltage drop. You may have bad connections alow. If you must stay stock then you may need to replace all wiring components.
If my vehicle I take out all in line connectors where I can do western union splace and solder then tape it.
Dan Bentler
