Current (Ampere) = Voltage/ Resistance
I (Ampere) = V / R
https://circuitdigest.com/tutorial/dc-circuit-theory
12v / 26 ohms = 2.166666666666667 current in circuit
to calculate voltage drop you will need the size and length of wire.
https://www.jcalc.net/voltage-drop-calculator-nec
To measure Voltage drop with meter hold the probes across the connection where you’re trying to measure the voltage drop, and energize the circuit. ie. put the red and black probe leads at two different points on the same side of the circuit, measure between the starting point, (switch) and ending point, (lamp).
find the spots with the largest voltage drop, clean / repair and re-test.
https://www.hagerty.com/media/maintenance-and-tech/measuring-voltage-drop/