Q: When trying to start it only clicks on a large relay (?ECU?) under the dash passenger side.

Often times when there is a starting issue, there is a small part called a starter relay that is many times over looked that is usually a fairly simple and inexpensive fix. The starter relay is a small unit near the starter that directs power to the starter from the battery. When this fails, it may cause a clicking sound and will not allow the starter to engage the flywheel when you turn the key. I would suggest starting with this first before going into a much more expensive repair like an ECU. If you would like to have this taken care of, a qualified professional from YourMechanic can diagnose the starting problem and guide with proper repairs.