It’s a long time since I’ve had my hands on a mk1. If your able to read the blink code it’s the correct led.

Does your car have the Obd1 or Obd2 plug that you can not connect to ?

i would think you have a wiring fault due to its age. The early Zetec’s really suffered with crumbling insulation and corrosion of the conductor, the Black Death.

With a multimeter and the ignition in the run position and the ecu removed. Check you have 12v+ on the one permanent live and two switched lives.
Again 12v+ at the transceiver.

check the earths have continuity to earth. Again with the transceiver.

The gems modual cuts some earth paths as part of the active anti theft so if you have a relay in this that has failed it could cause this.

Last Mondeo ecu I worked on was the mk3 Mondeo St220 over the summer.