Paul,

It is on the drivers side inner fender well, up near the fuse box but on the engine side of the second firewall. It should have a boost line coming from the intake or turbo side ( forget which) to the switch, then to the Alda. The alda is buried under the intake behind the IP. The overboost switch also has a set of wires going to it. Many folks here have SDL's and may be able to take a picture of it for you.

You can directly connect the boost signal to the alda ( bypass the switch). It is not recommended for a long term solution, but for testing if this is the problem you will know right away. When mine was bypassed I went from the 19-20 second 0-60 to a 12-13 second 0-60 time. The boost really came on in the 2500-2800 rpm range.

This is a reasonably common failure ( oil or crud in the lines, leaks, or a bad switch). There are of course multiple other areas to check. The Alda itself could be bad, the overboost valve on the turbo could be leaking.... but you can try the switch for $0 and 10 minutes of effort.

Good luck! I went from cursing the slow SDL to zooming and loving it.

Chuck