The BMW plug for the Hi Line system is shown in this picture.
By connecting a lead that converts the BMW plug to ISO connectors should be all you need to connect your new double din unit for power, steering wheel control and speakers.
The aerial would at the start of the conversion be plugged into the radio module [in the boot]. Unplug the aerial wire and extend the cable to behind the dashboard and connect to you double din unit.
At the start of installation the steering wheel controls operate from a small white module located behind the TV head unit.
You may have to remove this to enable your double din unit to fit.
I am almost certain that Harmon Kardon system will still be in the circuit and will fully operate with your new system.
When Rover and MG Rover built the 75 and installed the wiring loom they used one loom for the radio system regardless wether the hi line/tv system or Symphony / Harmony radios were fitted.
The Harmony radio was connected straight to the BMW plug shown in the picture.
The Symphony radio used a converter lead, BMW to ISO.
The hi line system used a BMW male to BMW female to connect the modules in the boot to the car loom [car loom/BMW plug shown above]
Some later model 75's both mk 1 and facelift did away with the BMW plug [shown above] and used male and female ISO connectors.
There is no way of knowing which type [BMW or ISO] would be fitted except by having a look.
This is because there was a great deal of using up existing stocks of looms,
not until the Harmony radio stopped being used was the change in connectors accelerated.
Dave T