There is another way, but it involves programming a microprocessor and designing a suitable electronic circuit.
What I would do is use a micro processor e.g. a PIC, Atmel or similar, with an analogue to digital converter and at least 6 usable outputs.
This gets fairly technical now, sorry
So, using a reference voltage have the 166 switches connected to the analogue to digital converter in the PIC ( or other processor ) to form a voltage divider.
The PIC, using interrupts ( preferably ) reads the ADC and works out which of the 6 buttons has been pressed. The PIC will then switch on an output which will in turn trigger one transistor in an array ( you don't want to connect the 156 head unit directly to the PIC ) and that will effectively send a signal to the 156 head unit via a suitable resistor. The output gets switched off when the button is released ( again controlled by interrupt ).
This interface can live behind the dashboard and the 166 steering wheel and clockspring remain standard.
Guess what I'm working on at the moment
( but not to a 156 head unit ).
By the way, you do know that the airbag connector on the 166 clockspring ( steering column side ) is different to the one on the GTV/Spider?
Apologies for getting heavily technical there.