Is the 12v switch rated for the current?
When you say "25a" cable, what size and type is the cable?
Is it 2.5mm twin and earth you have used?
Have you actually got a multimeter? have you done any voltage measurements?
What is the voltage measured at the first lamp?
What is the voltage measured at the last lamp?
For now take the PIR out of the equation, bypass it until you can sort out adding a relay/contactor capable of the current.
(You can put it in a waterproof box next to the current PIR)
You wire the contactor in so that you have the 12v going to the PIR as it is but the output of the PIR goes to the A1 terminal of the contactor.
The 0V goes to the relay A2.
The +12v supply to the lamps goes to the contactor common (L1) and the N/O output (T1) then goes to the lamps.
The main 0v goes to the lamps.
Instead of a contactor you could use an automotive relay, these are designed for high currents and of course are 12v.
In which case they are different pin numbers.
However, you really do need to be using a meter to measure the voltage at the lamps.
You may have volt drop along the cable & this could explain why it's not working.