Well, to avoid 'cross over' side effects when installing LEDs, here are a few pointers you may want to consider before going hog wild w/ LED installs.
*The LIGHT EMITTING DIODE (LED) itself has a very low resistance, above initial voltage ("ON") threshold. Therefore, most all of the LEDs intended as replacements for regular bulbs are packaged with a
series resistor to limit the current to prevent the LED itself from destruction. However, many circuits in (C4s) depend on a lower resistance, e.g., standard incandescent bulbs, to operate. and without the current draw resulting from a lower resistance (than replacement LED circuits have), all sorts of unintended (or 'cross-over') problems can occur.
So, there are solutions to keeping all the indicator lights, e.g., high beam indicator light, fog lites on (or off), blinker lights, turn signals, etc. operating.
Load Resistors wired across the input leads to the LED conversion will restore the normal current/voltage levels present before the LED(s) were installed. OR, installing a separate ground from the light to ground too can alleviate requirements for a load resistor while having the added advantage of reducing the DC load to the (alternator/battery) circuit.
Special switches (e.g., LED blinker switches in lieu of the standard (C4) stock blinker switches) are very helpful in avoiding issues with all of the turn signal bulb conversions, with or without LOAD resistors.
Nothing at all, in a few cases is necessary for a number of "dead-end" bulbs whose shared circuitry is NOT affected by the current/voltage changes installing certain LEDs will cause. For example, many of the LED replacement interior lights can be installed w/o any special consideration.
Below is the (orange) 25Ω load resistor across input to the FOG light that prevents in-cabin indicator light issues.
The cylinder can is the stock turn signal blinker. Replacing it with an LED (black (rectangular)) 2-pole switch allows the turn signals to operate normally
w/o load resistors.