12v LED Turn on and Dimming based on either MAP signal, or RPM of Car
AFAIK MAP sensors are available in two types - Analog and digital. Analog will give you an output voltage proportional to the pressure applied. Digital will give you a frequency proportional to the pressure. For our convenience (and tons of extra features), let's use a micro-controller (preferably arduino).

Analog MAP sensor: If the voltage is different than 0V-5V then first you need to convert the output signal to 0V - 5V output. Once you got 0-5V range signal, connect it to the analog pin of your arduino. From there, you will be able to get a value proportional to the output signal of MAP sensor.

Digital MAP sensor: If the voltage range is 0-5V, nice otherwise use an opto-coupler to do the logic level conversion. Something like this:



Connect the output to digital pin 2 of arduino. Now you can use interrupt based coding to figure out how many signal pulses you received per second (or per 10 sec). This again will give you a number proportional to the output signal of MAP sensor.

Now you have a number. When the number is high, you need the LEDs to light brighter or maybe blink at a fast pace and when the number is small, the light will glow dim or maybe you can add a slow breathing pattern.

To achieve this, get rid of electro-mechanical relay if you are using one and get a mosfet/power transistor which can handle the current required by your LEDs.

Read something about PWM.

Now all you need to do is to vary the duty cycle on the pin driving the mosfet/transistor. This will give you LED brightness proportional to the MAP signal output.

You can be creative and implement flashing lights, breathing lights or any other effect that you wish. Use RGB LED strip to get even more creative.