Congrat on ur new motor! Do u mean the light switch for the instrument panel? It maybe just loose connection. I had it on my Infiniti and I correct it by just shaking the stearing column. Now I HAVE NO IDEA IF this WILL WORK ON UR SL SO U MIGHT WANT TO WAIT FOR OTHERS (MORE EXPERIENCED MEMBERS)' RESPONSES.
Regarding the hissing sound, it might be the sound from extra air intake. I notice on my 55 that once rev go past about 3000rpm there will be this "air sucking sound" if u listen carefully.
Just my $0.02,