Welcome to the forum.
Not alot to go on but weak lights and slow blinker with engine off means battery voltage is too low, that could be the battery itself or the connections from battery to vehicle(battery cables).
People often check positive battery cable but not negative, both need to be clean connections for system to work properly
Car battery voltage should be 12.3 to 13 volts engine off.
Engine running, alternator working, should show battery voltage at 13.5 to 15 volts
Under 12 volts with engine running means alternator system is not working and engine can run rough because spark voltage is too low.
Turning on head lights drops voltage even lower and engine runs rougher.
So it reads like you have a charging system problem.
You don't mention the Battery Light in the dash?
Is it on with key on, so you know bulb is good?
And is it off after engine starts?
Check fuses #9(5amp) and #10(10amp) in cab fuse box, these "turn on" the alternator
Ford started using "smart charging" which means battery used should be silver calcium type, not standard lead acid type.
Test battery voltage with engine off and then with engine running
If they are close to the same then, unplug the 2 or 3 wire connector on alternator, smaller wires not the big wire
Check battery voltage again
If voltage is higher now then shut off engine, not good to run it this way.
Trace the 2 or 3 wires back to ECU they are probably damaged
Not an uncommon issue
Video of damaged control wires here: www.youtube.com/watch?v=lucj_ALV5M8