Retrofitting a coolant temperature gauge

This is rather easy on the Series II models (but read on, we have some ideas for Series I, too). The modification has been done on various models already: 15 TGE, 19 TRI, 19 TRS, 17 TZD turbo, 19 RD, 19 TZD. As far as I know, the exact same procedure should apply to all BX 15/16s (but it would be nice if someone could try and inform me about the results). Part of the modification might work on a BX 14, too (temperature meter but no warning lamp).