IF your intercom unit can be inside and you can hard-wire your mic input and speaker output from the SIP Intercom device, here is what we use for this same scenario:
Speaker:
https://smile.amazon.com/Indoor-Outdoor-Horn-Speaker-Impedance/dp/B002UL0WH8
Mic:
https://smile.amazon.com/Mono-Stereo-Parabolic-Microphone-Standard-060-Polycarbonate/dp/B00GGX3X7W
Because it is parabolic, it doesn't have to be close to the subject. We install them about 10' feet away.
These mics are so focused, we don't hear any engine noise, only the driver.
We build a gable roof (roof only) over the dish and the roof has only one end. We place the dish at the far end (the capped end) from the vehicle. The roof is sized so the disk is completely up/inside the roof line. The open end of the roof is at the vehicle side of the setup.
The mic array detects sound from air flow through the mic element, so you have to protect the mic array inside the dish from unfriendly weather while still allowing sound vibrations to get to the mic itself. This is why the roof with an open end. We used to cap both ends and cut a 22" circle at the vehicle end, but, for the extra work, it didn't really provide any additional protection from the elements.
Eventually the mic gets ruined. So we just replace the mic, while using the same dish, mount, and roof.
Wildtronics has a weatherproof mono parabolic coming out sometime next year. When that happens, you can do away with the roof.
The black dishes are less expensive that the clear dishes.
