[Old Davy]

Have had my mast out of the motor casing several times during mast rebuild refurbishment.
The black plastic (slightly rubbery) grommet ring sits fixed at the bottom of the main curved plastic guide tube where it secures to the plastic motor casing. When you pull the old mast and white 'cord' cable out - probably black and filthy - sometimes it brings the black grommet gasket with it - because of accumulated grease and oil and dirt. The grommet is the last line of defense to resist water entering the drum.
I don't think the how-to guides explain this fully...the black grommet should stay in place at the bottom.

Pop the grommet off the old mast cable.

You can remove that lower tube (if you remove the antenna assembly from the fender) and place the cleaned black grommet back in position, re-secure the curved mast guide tube, re-install the antenna assemble without the mast and proceed as usual. You could also insert the new mast cable with the assembly laying flat in the trunk with enough clearance for the mast to fully extend during reset - very important- then re-install the assembly in the fender.

As instructed it is best if you did not run the motor after removing the mast; makes replacing a new mast a 5 minute job.
Feed the new mast cable down the tube and forcefully pop the ball through the grommet, where it will enter the reel drum slot. It doesn't take much to pop the ball through the rubberized grommet - you will feel it pop in.

If you did run the antenna motor after pulling the cable the drum slot will be out of alignment. Run the antenna up and down a few times (with a friend's help) and eventually it will catch & fully retract. (The drum reel has a sort of a clutch that will reset itself to the correct position after a few up&downs.)

Removing the antenna from the fender, removing the aluminum tube and flushing out the tube with de-greaser is always a good idea as the tube will be fouled with old accumulated dirt mud and grease. No need to take apart the black plastic motor housing, btw.

One more thing - the purpose of the grommet is to center & guide the cable as it ups&downs to prevent it from scraping where it enters the drum. Will make the nylon cable last longer and avoid shavings inside the motor unit.

I rarely use the retracting antenna except for demonstration. The window glass grid antenna works fine by itself for AM & FM reception. I installed a power cut-off switch inside the trunk's rear marker bulb access panel.

So, tell us, please...where did you source a new OEM SVX mast?