Modifying El Wire driver for Brightness control
I assume you are reverse-engineering the circuit? Without a full schematic, it's hard to figure out an elegant solution.

I also assume that the subcircuit you show is a low-voltage control signal. If it is low-voltage, you could just use a digital potentiometer.

There are many styles of digital pots. They can be controlled by logic, serial (SPI or I2C), etc. They come in different packages, with different resistance ranges and resolutions.

As an example, please see Microchip's MCP4017. They can be found for around $0.50 USD in single quantities: