For what it's worth - I wanted to test this before hacking up my steering column, and I couldn't find details on the white connector (by the headlight adjuster).
You'd need to use a on-off-on latching switch, and the pinouts of the connector (looking from the bottom) are as follows :
| | X|
| L| |
Where ## is the missing pin, X is the common wire, C
is Cruise Control mode, and L is limit mode.
Also, for what it's worth, it all worked, and I went down the 'extra radio stalk' route, but I decided to stuff all the resistors into the stalk, and snip some tracks. My stalk (mounted on the opposite side to the radio) has Vol+ as Speed-, Vol- as Speed+, Mute as Restore, and the top and bottom buttons all as Cancel).
£10 in a local breakers for a spare radio stalk and a handful of resistors later, and I'm very happy (apart from the lack of daskboard lights - but I don't have a clip, and I'm don't want to pay £100 for the privilege). Is there an easy way??
On thing I did notice though is that the speed limiter 'kickdown' function doesn't appear to work - it doesn't matter how hard I stand on the accelerator, it won't budge over the limit unless you disengage SL.