Every module on a CAN BUS must have an address. Otherwise it is an unkniwn entity that the system does not recognize. There are other factors put into play when cruise control is introduced. It involves engine, transmission and braking so I doubt that you can jus plug something random in and it just listens. In order for the vehicle to respond, the transceivers need to be in communication with each other and understand each others language. Otherwise the other modules will ignore it and set a code. A module that sits there and talks to itself is known s a lonely node. I may be wrong but I have studied a lot of CAN BUS. Like I said, I dont know the car or the add on so I could be in left field.
You do not need an address for a module that just listens. However, to make use of the data, you need to find out the address and position within the data of the information that is received.
You can buy loggers that will receive on the CAN. I use one from Microchip.
Here is a single CAN frame that I received from my car, a 2009 Jaguar X-type:-
4B0 8 2A 73 2A BC 2A 30 2A 62
That is a frame with an ID (address) of 0x4B0 and it's 8 bytes long. That particular frame contains the four wheel speeds. Each is two bytes, and they are all in the range 0x2A30 to 0x2ABC when that particular frame was sent. I compared the log of that frame against GPS speed for some distance and worked out the speeds are in kph, in units of 0.01 kph. The four wheel speeds in that frame were in the range 108.00 to 109.40 kph.
I later worked out which speed was for each wheel by jacking the car and spinning the wheels by hand and seeing which bytes of the 0x4B0 frame changed.
I suspect that the device fitted by the OP is looking for the wrong CAN address, or in the wrong place within the 8 bytes, so it can't work out what the speed is.
Many of the CANbus readers that are available to read fault codes and stuff like that work rather differently. Fault codes, (DTCs) are not sent on the CANbus unless diagnostic equipment asks for them using a diagnostic request. In normal running on a car, there are no diagnostic requests.
When a DTC is read, the reader has to send a diagnostic request for the information, and the address that request is sent on will be for a particular module. That module will reply with the information. The reply can be several frames long.
It's possible to read things like vehicle speed in the same way, by asking a module using a diagnostic command. Many aftermarket devices do that, because the diagnostic commands are defined by international standards, and don't change from one make of car to another, or with model year, nearly as much as the the simple CAN data changes.
If a device is transmitting diagnostic requests on the CANbus, all sorts of things can go wrong that the rest of the car can detect, and possibly set a DTC. Different manufacturers will have different rules about what diagnostic requests are made at what times. If the device that is trying read data on the CANbus doesn't work correctly, it will interfere with other messages. I would never assume that aftermarket devices will stick to the rules about transmitting on the CANbus.
I have seen modules that transmit with nothing listening to them, what you call "lonely nodes". There is no problem with having CAN messages like that. Every time diagnostic equipment scans all the modules on a car, some of the optional ones may be missing. The diagnostic equipment just sends a message, receives no reply, and moves on to the next in the list.