You should wire the spotlights through an accessory relay, which has a pair of heavy contacts for the load current and a solenoid (coil) to close/open those contacts. Taking the load current from the exsisting wiring will stress the cables. From memory, see below the wiring for the relay.
30 - Fused power in...from battery +12v.
87 - Power out to spotlights +12v when relay closed.
85 - Trigger wire...from main beam wiring, +12v when main beam on.
86 - Earth...tie to earth locally.
Also earth each spotlamp locally, and use heavy enough cable to carry load (2.5mm)
Hope that helps.