Hello,
I have tried my best to answer all questions below.
1.) Is the fused "Parking Light Relays" pre-existing in the OEM F150 wiring?
- No, it is a relay you must purchase and add into the installation yourself.
1a.) If so, where is it located (it's a short wire and will not reach the passenger side)?
- See previous answer.
1b.) If not, can you please suggest a part number for the relay and fuses?
- Any standard automotive 5 pin relay will work. Fortin also sells one. https://fortin.ca/en/products/accessories/mr20/
1c.) Is there already an in-line 7.5 A fuse built into the t-harness for the brown/white parking light wire?
- No this is a fuse you must add when making the connections. You will need a 7.5A and a 10A as shown in the diagram you have linked above.
2.) Other posts mention connecting to the light switch (yellow/orange) instead of the passenger BCM. The wire is not long enough to reach the passenger side - is it designed to connect to the light switch on the driver's side?
- No, simply follow the relay diagram and connection location provided in the installation guide for your vehicle linked in your post above.
2a.) If the light switch is a better method, is an additional relay & fuses still needed (per above)?
- See previous answer.
2b.) If additional wire length is required to connect at the passenger's side, what is the wire gauge size?
- Simply match the gauge of the wire you are connecting it to.
Best Regards.