GU Patrol rear Lights

I have wired up both stop and tail lights and also R/H side reversing light into previous blank lights and have been ok for 2 months
Towed trailer last week and noticed that both Stop light globes have blown
Checked wiring and is definitely globes that have blown
Tail lights and reverse ligts are OK

I remember reading that it may be necessary to wire a relay up to these

If this is the case can anyone enligten me to the procedure for doing this

Otherwise I will just replace globes and see how they go next time

Thanks in advance