Speedometer cable lubrication

I would like to lubricate my speedometer cable (85-Z28 manual transmission). Shop manual states:

"On models without cruise control, disconnect the speedometer cable strap at the power brake booster."

Where is this strap? I can't find it anywhere on the brake booster. What does it look like...is it the rubber plug that goes into the firewall?
Is necessary to disconnect this strap?

Any tips would be appreciated.

I generally remove the instrument cluster from the dash, and push the clip in the back of the speedo head that holds the cable sheath in place, pulling it out the back. I then pull the cable from the sheath (have some old towels placed around so that the dirty cable won't stain anything in the cockpit), and clean it good, then I use wheel bearing grease to lubricate the cable as I'm putting it back into the sheath.

yeah you might even be able to get the cable off without removing the cluster by just taking off the acces panel and just reaching up there. Thats how i did it on my red car. It just goes right through the firewall and then routes down to the trans and plugs into the tailshaft. If you dont see a strap dont worry about it. The cable just follows the frame rail all the way down. Then like silcon said use wheel bearing greese or any high speed greese as long as its not too heavy or too liquidy. Yes thats a word....its my word..so there.... :)

Thanks for the great tips!

I did find that so called "strap". It was a bracket holding the cable. Once I loosened it, I was able to pull more of the cable into the interior and disconnect it from the speedo.

Lubed it up with your recommendation of wheel bearing grease. All good now.