It's unusually lucky for light-dimming to variably work properly with an aftermarket radio that provides only one variable light-dimming control input.
That's because the Miata's light dimming control signal is an inverted one. It's customary for aftermarket radios with only one variable light-dimming control input to be set up for a non-inverted control signal that many other cars use. There are some aftermarket radios that have two variable light-dimming control inputs, one for an inverted control signal from the car, and another input for a non-inverted one.
It's that less common inverting input that's needed for Miatas. Without that inverting input when the Miata's dash dimming control sets the car's dash lights brighter, the aftermarket radio's lighting would become dimmer, instead of brighter along with the car's dash lights, and ashtray light.
Nevertheless reversed adjustable brilliance lighting operation, it may be possible to produce an illusion of adequate operations. That's because when the Miata's dash dimmer control is set, for example, to half dash-light brilliance, the opposite light-brilliance response from the aftermarket radio may also be at its point of half brilliance too (or possibly at least close enough to be usably dimmed).
The NA Miata's TNS (The Nighttime Signal) is the red/black wiring, and the dimming signal's is the red wiring, both at the dash dimmer control (and also at the ashtray light). When the Miata's dash light dimming control is not adequate for dimming an aftermarket radio's lighting, the Miata's TNS signal might be suitable for switching in the aftermarket radio's night lighting mode.
The Miata's TNS signal is +12V with its nighttime selection (i.e., with parking lights turned on). The Miata's dimming signal approximates +0V when the dash dimmer is set to brightest, and approximates +12V when set dimmest. However, that's given just for simply comprehending. The Miata's dash dimming signal is not actually voltage variable. Instead, the Miata's dimming signal is a variable pulsed-width one that adjusts electrical pulses of variable durations for the appearance of light dimming.
For NA Miata's, lighting for aftermarket accessories can conditionally be wired between that red/black and red wiring for adjustable lighting brilliance that properly works. That's not always the case, for example, when the aftermarket accessory's lighting includes a common ground. It's routine for lighted aftermarket accessories to have a common ground for their lighting, including an electrical ground the may be established by mounting the accessory to the car. Given enough effort for physical reconstruction, that common grounding can most times be eliminated.
Here's a link to a recent Thread related to Miata light dimming:
http://forum.miata.net/vb/showthread.php?t=363009
