There are a couple of outputs on the 20 pin connector that will do what you need.
First, there is a standard "external" speaker connection that is controlled by the volume knob. This is on pins 1 and 16. Pin 1 is speaker (-), Pin 16 is speaker (+). Do not ground either of these leads, used a dedicated pair of wires directly to the speaker.
Second, there is "flat RX audio" on pin 11. Use that along with pin 7 (ground) to feed your amplifier.
I bought a bunch of the connectors from a guy off e-bay. You can either buy the kit with the pins and the connector body, or you can buy "pre wired" sets that have the pins already crimped on and a connector body. You may have an easier time finding the 16 pin connector body, it's a lot more popular. It will work in the 20 pin socket of the CDM radios just fine. The 2 pins on either side are not necessary for what you are doing.
You can find the pinout information for the CDM radios here: HT750/1250/1550/PRO5150/7150/9150/GP320/340/360/380/ GP318/338/GP640/680/1280/CDM750/CDM1250/CDM1550,etc.
Connectors: check e-bay for a guy with users name MRE1032. This listing would give you what you need for one radio: Kit 1 Accy Plug 16 Pin Motorola Maxtrac GM300 Repeater | eBay