Hi Luka,
It depends what engine you have.
1.2, and 1.4 have the ECU on the passenger side of the bulkhead/firewall.
1.6 has it mouted on the battery tray next to the engine.
1.8 Mk1 and 2.0 Mk1 had it mounted in car behind the drivers side fuse panel.
1.8 Mk2 Is mounted on the back of the inlet manifold
2.0 Mk2 its mounted infront of the power steering fluid tank
Whats the problem exactly? we should be able to give you some good advice.