window.onload = function () {
    var prev = document.querySelector('input[data-direction="prev"]');
    var next = document.querySelector('input[data-direction="next"]');

    rotator.initButtons(prev, next);
// ....
}

var rotator = rotator || new ImageRotator(".splash > img", "data-index", "visible", "hidden", ".details > img", ".details > div");

// constructor
function ImageRotator()
{
    // ....

        this.initButtons = function (prev, next) {

        prev.onclick = function () {
            that.rotateLeft();
        };

        next.onclick = function () {
            that.rotateRight();
        };

        _prevButton = prev;
        _nextButton = next;

        _refreshContent();
    };
}