Why not use the headphone jack that is already on the front of the scanner as noted on page 10 and 11 in the manual that came with the radio?
From the manual:
CONNECTING AN EXTENSION SPEAKER
In a noisy area, an amplified extension speaker (such as Cat. No. 21-541), positioned in the right place, might provide more comfortable listening. Plug the speaker cable’s 1/8-inch plug into your scanner’s headphone jack.